CHAPTER XIV - MUTUAL AFFINITIES OF ORGANIC BEINGS:
MORPHOLOGY: EMBRYOLOGY: RUDIMENTARY ORGANS
Classification
FROM the most remote period in the history of the world organic
beings have been found to resemble each other in descending degrees,
so that they can be classed in groups under groups. This
classification is not arbitrary like the grouping of the stars in
constellations. The existence of groups would have been of simpler
significance, if one group had been exclusively fitted to inhabit
the land and another the water; one to feed on flesh, another on
vegetable matter, and so on; but the case is widely different, for
it is notorious how commonly members of even the same subgroup have
different habits. In the second and fourth chapters, on Variation
and on Natural Selection, I have attempted to show that within each
country it is the widely ranging, the much diffused and common, that
is the dominant species, belonging to the larger genera in each class,
which vary most. The varieties, or incipient species, thus produced,
ultimately become converted into new and distinct species; and
these, on the principle of inheritance, tend to produce other new
and dominant species. Consequently the groups which are now large,
and which generally include many dominant species, tend to go on
increasing in size. I further attempted to show that from the
varying descendants of each species trying to occupy as many and as
different places as possible in the economy of nature, they constantly
tend to diverge in character. This latter conclusion is supported by
observing the great diversity of forms which, in any small area,
come into the closest competition, and by certain facts in
naturalisation.
I attempted also to show that there is a steady tendency in the
forms which are increasing in number and diverging in character, to
supplant and exterminate the preceding, less divergent and less
improved forms. I request the reader to turn to the diagram
illustrating the action, as formerly explained, of these several
principles; and he will see that the inevitable result is, that the
modified descendants proceeding from one progenitor become broken up
into groups subordinate to groups. In the diagram each letter on the
uppermost line may represent a genus including several species, and
the whole of the genera along this upper line form together one class,
for all are descended from one ancient parent, and, consequently, have
inherited something in common. But the three genera on the left hand
have, on this same principle, much in common, and form a sub-family,
distinct from that containing the next two genera on the right hand,
which diverged from a common parent at the fifth stage of descent.
These five genera have also much in common, though less than when
grouped in sub-families; and they form a family distinct from that
containing the three genera still farther to the right hand, which
diverged at an earlier period. And all these genera, descended from
(A), form an order distinct from the genera descended from (I). So
that we here have many species descended from a single progenitor
grouped into genera; and the genera into sub-families, families, and
orders, all under one great class. The grand fact of the natural
subordination of organic beings in groups under groups, which, from
its familiarity, does not always sufficiently strike us, is in my
judgment thus explained. No doubt organic beings, like all other
objects, can be classed in many ways, either artificially by single
characters, or more naturally by a number of characters. We know, for
instance, that minerals and the elemental substances can be thus
arranged. In this case there is of course no relation to genealogical
succession, and no cause can at present be assigned for their falling
into groups. But with organic beings the case is different, and the
view above given accords with their natural arrangement in group under
group; and no other explanation has ever been attempted.
Naturalists, as we have seen, try to arrange the species, genera,
and families in each class, on what is called the Natural System.
But what is meant by this system? Some authors look at it merely as
a scheme for arranging together those living objects which are most
alike, and for separating those which are most unlike; or as an
artificial method of enunciating, as briefly as possible, general
propositions,- that is, by one sentence to give the characters common,
for instance, to all mammals, by another those common to all
carnivora, by another those common to the dog-genus, and then, by
adding a single sentence, a full description is given of each kind
of dog. The ingenuity and utility of this system are indisputable. But
many naturalists think that something more is meant by the Natural
System; they believe that it reveals the plan of the Creator; but
unless it be specified whether order in time or space, or both, or
what else is meant by the plan of the Creator, it seems to me that
nothing is thus added to our knowledge. Expressions such as that
famous one by Linnaeus, which we often meet with in a more or less
concealed form, namely, that the characters do not make the genus, but
that the genus gives the characters, seem to imply that some deeper
bond is included in our classifications than mere resemblance. I
believe that this is the case, and that community of descent- the
one known cause of close similarity in organic beings- is the bond,
which though observed by various degrees of modification, is partially
revealed to us by our classifications.
Let us now consider the rules followed in classification, and the
difficulties which are encountered on the view that classification
either gives some unknown plan of creation, or is simply a scheme
for enunciating general propositions and of placing together the forms
most like each other. It might have been thought (and was in ancient
times thought) that those parts of the structure which determined
the habits of life, and the general place of each being in the economy
of nature, would be of very high importance in classification. Nothing
can be more false. No one regards the external similarity of a mouse
to a shrew, of a dugong to a whale, of a whale to a fish, as of any
importance. These resemblances, though so intimately connected with
the whole life of the being, are ranked as merely " adaptive or
analogical characters "; but to the consideration of these
resemblances we shall recur. It may even be given as a general rule,
that the less any part of the organisation is concerned with special
habits, the more important it becomes for classification. As an
instance: Owen, in speaking of the dugong, says, "The generative
organs, being those which are most remotely related to the habits
and food of an animal, I have always regarded as affording very
clear indications of its true affinities. We are least likely in the
modifications of these organs to mistake a merely adaptive for an
essential character." With plants how remarkable it is that the organs
of vegetation, on which their nutrition and life depend, are of little
significance; whereas the organs of reproduction, with their product
the seed and embryo, are of paramount importance! So again in formerly
discussing certain morphological characters which are not functionally
important, we have seen that they are often of the highest service
in classification. This depends on their constancy throughout many
allied groups; and their constancy chiefly depends on any slight
deviations not having been preserved and accumulated by natural
selection, which acts only on serviceable characters.
That the mere physiological importance of an organ does not
determine its classificatory value, is almost proved by the fact
that in allied groups, in which the same organ, as we have every
reason to suppose, has nearly the same physiological value, its
classificatory value is widely different. No naturalist can have
worked long at any group without being struck with this fact; and it
has been fully acknowledged in the writings of almost every author. It
will suffice to quote the highest authority, Robert Brown, who, in
speaking of certain organs in the Proteaceae, says their generic
importance, "like that of all their parts, not only in this, but, as
apprehend, in every natural family, is very unequal, and in some cases
seems to be entirely lost." Again, in another work he says, the genera
of the Connaraceae "differ in having one or more ovaria, in the
existence or absence of albumen, in the imbricate or valvular
aestivation. Any one of these characters singly is frequently of
more than generic importance, though here even when all taken together
they appear insufficient to separate Cnestis from Connarus." To give
an example amongst insects: in one great division of the
Hymenoptera, the antennae, as Westwood has remarked, are most constant
in structure; in another division they differ much, and the
differences are of quite subordinate value in classification; yet no
one will say that the antennae in these two divisions of the same
order are of unequal physiological importance. Any number of instances
could be given of the varying importance for classification of the
same important organ within the same group of beings.
Again, no one will say that rudimentary or atrophied organs are of
high physiological or vital importance; yet, undoubtedly, organs in
this condition are often of much value in classification. No one
will dispute that the rudimentary teeth in the upper jaws of young
ruminants, and certain rudimentary bones of the leg, are highly
serviceable in exhibiting the close affinity between ruminants and
pachyderms. Robert Brown has strongly insisted on the fact that the
position of the rudimentary florets is of the highest importance in
the classification of the grasses.
Numerous instances could be given of characters derived from parts
which must be considered of very trifling physiological importance,
but which are universally admitted as highly serviceable in the
definition of whole groups. For instance, whether or not there is an
open passage from the nostrils to the mouth, the only character,
according to Owen, which absolutely distinguishes fishes and reptiles-
the inflection of the angle of the lower jaw in marsupials- the manner
in which the wings of insects are folded- mere colour in certain
Algae- mere pubescence on parts of the flower in grasses- the nature
of the dermal covering, as hair or feathers, in the Vertebrata. If the
Ornithorhynchus had been covered with feathers instead of hair, this
external and trifling character would have been considered by
naturalists as an important aid in determining the degree of
affinity of this strange creature to birds.
The importance, for classification, of trifling characters, mainly
depends on their being correlated with many other characters of more
or less importance. The value indeed of an aggregate of characters
is very evident in natural history. Hence, as has often been remarked,
a species may depart from its allies in several characters, both of
high physiological importance, and of almost universal prevalence, and
yet leave us in no doubt where it should be ranked. Hence, also, it
has been found that a classification founded on any single
character, however, important that may be, has always failed; for no
part of the organisation is invariably constant. The importance of
an aggregate of characters, even when none are important, alone
explains the aphorism enunciated by Linnaeus, namely, that the
characters do not give the genus, but the genus gives the
characters; for this seems founded on the appreciation of many
trifling points of resemblance, too slight to be defined. Certain
plants, belonging to the Malpighiaceae, bear perfect and degraded
flowers; in the latter, as A. de Jussieu has remarked, " The greater
number of the characters proper to the species, to the genus, to the
family, to the class, disappear, and thus laugh at our
classification." When Aspicarpa produced in France, during several
years, only these degraded flowers, departing so wonderfully in a
number of the most important points of structure from the proper
type of the order, yet M. Richard sagaciously saw, as Jussieu
observes, that this genus should still be retained amongst the
Malpighiaceae. This case well illustrates the spirit of our
classifications.
Practically, when naturalists are at work, they do not trouble
themselves about the physiological value of the characters which
they use in defining a group or in allocating any particular
species. If they find a character nearly uniform, and common to a
great number of forms, and not common to others, they use it as one of
high value; if common to some lesser number, they use it as of
subordinate value. This principle has been broadly confessed by some
naturalists to be the true one; and by none more clearly than by
that excellent botanist, Auguste de Saint-Hilaire. If several trifling
characters are always found in combination, though no apparent bond of
connection can be discovered between them, especial value is set on
them. As in most groups of animals, important organs, such as those
for propelling the blood, or for Aerating it, or those for propagating
the race, are found nearly uniform, they are considered as highly
serviceable in classification; but in some organs all these, the
most important vital organs, are found to offer characters of quite
subordinate value. Thus, as Fritz Muller has lately remarked, in the
same group of crustaceans, Cypridina is furnished with a heart, whilst
in two closely allied genera, namely Cypris and Cytherea, there is
no such organ; one species of Cypridina has well-developed
branchiae, whilst another species is destitute of them.
We can see why characters derived from the embryo should be of equal
importance with those derived from the adult, for a natural
classification of course includes all ages. But it is by no means
obvious, on the ordinary view, why the structure of the embryo
should be more important for this purpose than that of the adult,
which alone plays its full part in the economy of nature. Yet it has
been strongly urged by those great naturalists, Milne Edwards and
Agassiz, that embryological characters are the most important of
all; and this doctrine has very generally been admitted as true.
Nevertheless, their importance has sometimes been exaggerated, owing
to the adaptive characters of larvae not having been excluded; in
order to show this, Fritz Muller arranged by the aid of such
characters alone the great class of crustaceans, and the arrangement
did not prove a natural one. But there can be no doubt that
embryonic, excluding larval characters, are of the highest value for
classification, not only with animals but with plants. Thus the main
divisions of flowering plants are founded on differences in the
embryo,- on the number and position of the cotyledons, and on the mode
of development of the plumule and radicle. We shall immediately see
why these characters possess so high a value in classification,
namely, from the natural system being genealogical in its arrangement.
Our classifications are often plainly influenced by chains of
affinities. Nothing can be easier than to define a number of
characters common to all birds; but with crustaceans, any such
definition has hitherto been found impossible. There are crustaceans
at the opposite ends of the series, which have hardly a character in
common; yet the species at both ends, from being plainly allied to
others, and these to others, and so onwards, can be recognised as
unequivocally belonging to this, and to no other class of the
Articulata.
Geographical distribution has often been used, though perhaps not
quite logically, in classification, more especially in very large
groups of closely allied forms. Temminck insists on the utility or
even necessity of this practice in certain groups of birds; and it has
been followed by several entomologists and botanists.
Finally, with respect to the comparative value of the various groups
of species, such as orders, sub-orders, families, sub-families, and
genera, they seem to be, at least at present, almost arbitrary.
Several of the best botanists, such as Mr. Bentham and others, have
strongly insisted on their arbitrary value. Instances could be given
amongst plants and insects, of a group first ranked by practised
naturalists as only a genus, and then raised to the rank of a
sub-family or family; and this has been done, not because further
research has detected important structural differences, at first
overlooked, but because numerous allied species with slightly
different grades of difference, have been subsequently discovered.
All the foregoing rules and aids and difficulties in
classification may be explained, if I do not greatly deceive myself,
on the view that the Natural System is founded on descent with
modification;- that the characters which naturalists consider as
showing true affinity between any two or more species, are those which
have been inherited from a common parent, all true classification
being genealogical;- that community of descent is the hidden bond
which naturalists have been unconsciously seeking, and not some
unknown plan of creation, or the enunciation of general
propositions, and the mere putting together and separating objects
more or less alike.
But I must explain my meaning more fully. I believe that the
arrangement of the groups within each class, in due subordination
and relation to each other, must be strictly genealogical in order
to be natural; but that the amount of difference in the several
branches or groups, though allied in the same degree in blood to their
common progenitor, may differ greatly, being due to the different
degrees of modification which they have undergone; and this is
expressed by the forms being ranked under different genera,
families, sections, or orders. The reader will best understand what is
meant, if he will take the trouble to refer to the diagram in the
fourth chapter.
We will suppose the letters A to L to represent allied genera
existing during the Silurian epoch, and descended from some still
earlier form. In three of these genera (A, F, and I), a species has
transmitted modified descendants to the present day, represented by
the fifteen genera (a14 to z14) on the uppermost horizontal line. Now
all these modified descendants from a single species, are related in
blood or descent in the same degree; they may metaphorically be called
cousins to the same millionth degree; yet they differ widely and in
different degrees from each other. The forms descended from A, now
broken up into two or three families, constitute a distinct order from
those descended from I, also broken up into two families. Nor can
the existing species, descended from A, be ranked in the same genus
with the parent A; or those from I, with the parent I. But the
existing genus f14 may be supposed to have been but slightly modified;
and it will then rank with the parent-genus F; just as some few
still living organisms belong to Silurian genera. So that the
comparative value of the differences between these organic beings,
which are all related to each other in the same degree in blood, has
come to be widely different. Nevertheless their genealogical
arrangement remains strictly true, not only at the present time, but
at each successive period of descent. All modified descendants from
A will have inherited something in common from their common parent, as
will all the descendants from I; so will it be with each subordinate
branch of descendants, at each successive stage. If, however, we
suppose any descendant of A, or of I, to have become so much
modified as to have lost all traces of its parentage, in this case,
its place in the natural system will be lost, as seems to have
occurred with some few existing organisms. All the descendants of
the genus F, along its whole line of descent, are supposed to have
been but little modified, and they form a single genus. But this
genus, though much isolated, will still occupy its proper intermediate
position. The representation of the groups, as here given in the
diagram on a flat surface, is much too simple. The branches ought to
have diverged in all directions. If the names of the groups had been
simply written down in a linear series, the representation would have
been still less natural; and it is notoriously not possible to
represent in a series, on a flat surface, the affinities which we
discover in nature amongst the beings of the same group. Thus, the
Natural System is genealogical in its arrangement, like a pedigree:
but the amount of modification which the different groups have
undergone has to be expressed by ranking them under different
so-called genera, sub-families, families, sections, orders, and
classes.
It may be worth while to illustrate this view of classification,
by taking the case of languages. If we possessed a perfect pedigree of
mankind, a genealogical arrangement of the races of man would afford
the best classification of the various languages now spoken
throughout the world; and if all extinct languages, and all
intermediate and slowly changing dialects, were to be included, such
an arrangement would be the only possible one. Yet it might be that
some ancient languages had altered very little and had given rise to
few new languages, whilst others had altered much owing to the
spreading, isolation, and state of civilisation of the several
co-descended races, and had thus given rise to many new dialects and
languages. The various degrees of difference between the languages
of the same stock, would have to be expressed by groups subordinate to
groups; but the proper or even the only possible arrangement would
still be genealogical; and this would be strictly natural, as it would
connect together all languages, extinct and recent, by the closest
affinities, and would give the filiation and origin of each tongue.
In confirmation of this view, let us glance at the classification of
varieties, which are known or believed to be descended from a single
species. These are grouped under the species, with the sub-varieties
under the varieties; and in some cases, as with the domestic pigeon,
with several other grades of difference. Nearly the same rules are
followed as in classifying species. Authors have insisted on the
necessity of arranging varieties on a natural instead of an
artificial system; we are cautioned, for instance, not to class two
varieties of the pineapple together, merely because their fruit,
though the most important part, happens to be nearly identical; no one
puts the Swedish and common turnip together, though the esculent and
thickened stems are so similar. Whatever part is found to be most
constant, is used in classing varieties: thus the great
agriculturist Marshall says the horns are very useful for this purpose
with cattle, because they are less variable than the shape or colour
of the body, &c.; whereas with sheep the horns are much less
serviceable, because less constant. In classing varieties, I
apprehend that if we had a real pedigree, a genealogical
classification would be universally preferred; and it has been
attempted in some cases. For we might feel sure, whether there had
been more or less modification, that the principle of inheritance
would keep the forms together which were allied in the greatest number
of points. In tumbler pigeons, though some of the sub-varieties differ
in the important character of the length of the beak, yet all are kept
together from having the common habit of tumbling; but the short-faced
breed has nearly or quite lost this habit; nevertheless, without any
thought on the subject, these tumblers are kept in the same group,
because allied in blood and alike in some other respects.
With species in a state of nature, every naturalist has in fact
brought descent into his classification; for he includes in his lowest
grade, that of species, the two sexes; and how enormously these
sometimes differ in the most important characters, is known to every
naturalist: scarcely a single fact can be predicated in common of
the adult males and hermaphrodites of certain cirripedes, and yet no
one dreams of separating them. As soon as the three orchidean forms,
Monachanthus, Myanthus, and Catasetum, which had previously been
ranked as three distinct genera, were known to be sometimes produced
on the same plant, they were immediately considered as varieties;
and now I have been able to show that they are the male, female, and
hermaphrodite forms of the same species. The naturalist includes as
one species the various larval stages of the same individual,
however much they may differ from each other and from the adult, as
well as the so-called alternate generations of Steenstrup, which can
only in a technical sense be considered as the same individual. He
includes monsters and varieties, not from their partial resemblance to
the parent-form, but because they are descended from it.
As descent has universally been used in classing together the
individuals of the same species, though the males and females and
larvae are sometimes extremely different; and as it has been used in
classing varieties which have undergone a certain, and sometimes a
considerable amount of modification, may not this same element of
descent have been unconsciously used in grouping species under
genera, and genera under higher groups, all under the so-called
natural system? I believe it has been unconsciously used; and thus
only can I understand the several rules and guides which have been
followed by our best systematists. As we have no written pedigrees, we
are forced to trace community of descent by resemblances of any
kind. Therefore we chose those characters which are the least likely
to have been modified, in relation to the conditions of life to
which each species has been recently exposed. Rudimentary structures
on this view are as good as, or even better than, other parts of the
organisation. We care not how trifling a character may be- let it be
the mere inflection of the angle of the jaw, the manner in which an
insect's wing is folded, whether the skin be covered by hair or
feathers- if it prevail throughout many and different species,
especially those having very different habits of life, it assumes high
value; for we can account for its presence in so many forms with
such different habits, only by inheritance from a common parent. We
may err in this respect in regard to single points of structure, but
when several characters, let them be ever so trifling, concur
throughout a large group of beings having different habits, we may
feel almost sure, on the theory of descent, that these characters
have been inherited from a common ancestor; and we know that such
aggregated characters have especial value in classification.
We can understand why a species or a group of species may depart
from its allies, in several of its most important characteristics, and
yet be safely classed with them. This may be safely done, and is
often done, as long as a sufficient number of characters, let them
be ever so unimportant, betrays the hidden bond of community of
descent. Let two forms have not a single character in common, yet,
if these extreme forms are connected together by a chain of
intermediate groups, we may at once infer their community of
descent, and we put them all into the same class. As we find organs of
high physiological importance- those which serve to preserve life
under the most diverse conditions of existence- are generally the most
constant, we attach especial value to them; but if these same
organs, in another group or section of a group, are found to differ
much, we at once value them less in our classification. We shall
presently see why embryological characters are of such high
classificatory importance. Geographical distribution may sometimes
be brought usefully into play in classing large genera, because all
the species of the same genus, inhabiting any distinct and isolated
region, are in all probability descended from the same parents.
Analogical Resemblances.- We can understand, on the above views, the
very important distinction between real affinities and analogical or
adaptive resemblances. Lamarck first called attention to this subject,
and he has been ably followed by Macleay and others. The resemblance
in the shape of the body and in the fin-like anterior limbs between
dugongs and whales, and between these two orders of mammals and
fishes, are analogical. So is the resemblance between a mouse and a
shrewmouse (Sorex), which belong to different orders; and the still
closer resemblance, insisted on by Mr. Mivart, between the mouse and a
small marsupial animal (Antechinus) of Australia. These latter
resemblances may be accounted for, as it seems to me, by adaptation
for similarly active movements through thickets and herbage,
together with concealment from enemies.
Amongst insects there are innumerable similar instances; thus
Linnaeus, misled by external appearances, actually classed an
homopterous insect as a moth. We see something of the same kind even
with our domestic varieties, as in the strikingly similar shape of the
body in the improved breeds of the Chinese and common pig, which are
descended from distinct species; and in the similarly thickened
stems of the common and specifically distinct Swedish turnip. The
resemblance between the greyhound and the race-horse is hardly more
fanciful than the analogies which have been drawn by some authors
between widely different animals.
On the view of characters being of real importance for
classification, only in so far as they reveal descent, we can
clearly understand why analogical or adaptive characters, although
of the utmost importance to the welfare of the being, are almost
valueless to the systematist. For animals, belonging to two most
distinct lines of descent, may have become adapted to similar
conditions, and thus have assumed a close external resemblance; but
such resemblances will not reveal- will rather tend to conceal their
blood-relationship. We can thus understand the apparent paradox,
that the very same characters are analogical when one group is
compared with another, but give true affinities when the members of
the same group are compared together: thus, the shape of the body
and fin-like limbs are only analogical when whales are compared with
fishes, being adaptations in both classes for swimming through the
water; but between the several members of the whale family, the
shape of the body and the fin-like limbs offer characters exhibiting
true affinity; for as these parts are so nearly similar throughout the
whole family, we cannot doubt that they have been inherited from a
common ancestor. So it is with fishes.
Numerous cases could be given of striking resemblances in quite
distinct beings between single parts or organs, which have been
adapted for the same functions. A good instance is afforded by the
close resemblance of the jaws of the dog and Tasmanian wolf or
Thylacinus,- animals which are widely sundered in the natural
system. But this resemblance is confined to general appearance, as
in the prominence of the canines, and in the cutting shape of the
molar teeth. For the teeth really differ much: thus the dog has on
each side of the upper jaw four pre-molars and only two molars; whilst
the Thylacinus has three pre-molars and four molars. The molars also
differ much in the two animals in relative size and structure. The
adult dentition is preceded by a widely different milk dentition.
Any one may of course deny that the teeth in either case have been
adapted for tearing flesh, through the natural selection of successive
variations; but if this be admitted in the one case, it is
unintelligible to me that it should be denied in the other. I am
glad to find that so high an authority as Professor Flower has come to
this same conclusion.
The extraordinary cases given in a former chapter, of widely
different fishes possessing electric organs,- of widely different
insects possessing luminous organs,- and of orchids and asclepiads
having pollen-masses with viscid discs, come under this same head of
analogical resemblances. But these cases are so wonderful that they
were introduced as difficulties or objections to our theory. In all
such cases some fundamental difference in the growth or development of
the parts, and generally in their matured structure, can be
detected. The end gained is the same, but the means, though
appearing superficially to be the same, are essentially different. The
principle formerly alluded to under the term of analogical variation
has probably in these cases often come into play; that is, the members
of the same class, although only distantly allied, have inherited so
much in common in their constitution, that they are apt to vary
under similar exciting causes in a similar manner; and this would
obviously aid in the acquirement through natural selection of parts or
organs, strikingly like each other, independently of their direct
inheritance from a common progenitor.
As species belonging to distinct classes have often been adapted
by successive slight modifications to live under nearly similar
circumstances,- to inhabit, for instance, the three elements of
land, air, and water,- we can perhaps understand how it is that a
numerical parallelism has sometimes been observed between the
sub-groups of distinct classes. A naturalist, struck with a
parallelism of this nature, by arbitrarily raising or sinking the
value of the groups in several classes (and all our experience shows
that their valuation is as yet arbitrary), could easily extend the
parallelism over a wide range; and thus the septenary, quinary,
quarternary and ternary classifications have probably arisen.
There is another and curious class of cases in which close
external resemblance does not depend on adaptation to similar habits
of life, but has been gained for the sake of protection. I allude to
the wonderful manner in which certain butterflies imitate, as first
described by Mr. Bates, other and quite distinct species. This
excellent observer has shown that in some districts of S. America,
where, for instance, an Ithomia abounds in gaudy swarms, another
butterfly, namely, a leptalis, is often found mingled in the same
flock; and the latter so closely resembles the Ithomia in every
shade and stripe of colour and even in the shape of its wings, that
Mr. Bates, with his eyes sharpened by collecting during eleven
years, was, though always on his guard, continually deceived. When the
mockers and the mocked are caught and compared, they are found to be
very different in essential structure, and to belong not only to
distinct genera, but often to distinct families. Had this mimicry
occurred in only one or two instances, it might have been passed
over as a strange coincidence. But, if we proceed from a district
where one Leptalis imitates an Ithomia, another mocking and mocked
species, belonging to the same two genera, equally close in their
resemblance, may be found. Altogether no less than ten genera are
enumerated, which include species that imitate other butterflies.
The mockers and mocked always inhabit the same region; we never find
an imitator living remote from the form which it imitates. The mockers
are almost invariably rare insects; the mocked in almost every case
abound in swarms. In the same district in which a species of
laptalis closely imitates an Ithomia, there are sometimes other
Lepidoptera mimicking the same Ithomia: so that in the same place,
species of three genera of butterflies and even a moth are found all
closely resembling a butterfly belonging to a fourth genus. It
deserves especial notice that many of the mimicking forms of the
leptalis, as well as of the mimicked forms, can be shown by a
graduated series to be merely varieties of the same species; whilst
others are undoubtedly distinct species. But why, it may be asked, are
certain forms treated as the mimicked and others as the mimickers? Mr.
Bates satisfactorily answers this question, by showing that the form
which is imitated keeps the usual dress of the group to which it
belongs, whilst the counterfeiters have changed their dress and do not
resemble their nearest allies.
We are next led to inquire what reason can be assigned for certain
butterflies and moths so often assuming the dress of another and
quite distinct form; why, to the perplexity of naturalists, has nature
condescended to the tricks of the stage? Mr. Bates has, no doubt,
hit on the true explanation. The mocked forms, which always abound
in numbers, must habitually escape destruction to a large extent,
otherwise they could not exist in such swarms; and a large amount of
evidence has now been collected, showing that they are distasteful
to birds and other insect-devouring animals. The mocking forms, on the
other hand, that inhabit the same district, are comparatively rare,
and belong to rare groups; hence they must suffer habitually from some
danger, for otherwise, from the number of eggs laid by all
butterflies, they would in three or four generations swarm over the
whole country. Now if a member of one of these persecuted and rare
groups were to assume a dress so like that of a well-protected species
that it continually deceived the practised eye of an entomologist,
it would often deceive predaceous birds and insects, and thus often
escape destruction. Mr. Bates may almost be said to have actually
witnessed the process by which the mimickers have come so closely to
resemble the mimicked; for he found that some of the forms of Leptalis
which mimic so many other butterflies, varied in an extreme degree. In
one district several varieties occurred, and of these one alone
resembled to a certain extent, the common Ithomia of the same
district. In another district there were two or three varieties, one
of which was much commoner than the others, and this closely mocked
another form of Ithomia. From facts of this nature, Mr. Bates
concludes that the leptalis first varies; and when a variety happens
to resemble in some degree any common butterfly inhabiting the same
district, this variety, from its resemblance to a flourishing and
little-persecuted kind, has a better chance of escaping destruction
from predaceous birds and insects, and is consequently oftener
preserved;- "the less perfect degrees of resemblance being
generation after generation eliminated, and only the others left to
propagate their kind." So that here we have an excellent
illustration of natural selection.
Messrs. Wallace and Trimen have likewise described several equally
striking cases of imitation in the Lepidoptera of the Malay
Archipelago and Africa, and with some other insects. Mr. Wallace has
also detected one such case with birds, but we have none with the
larger quadrupeds. The much greater frequency of imitation with
insects than with other animals, is probably the consequence of
their small size; insects cannot defend themselves, excepting indeed
the kinds furnished with a sting, and I have never heard of an
instance of such kinds mocking other insects, though they are
mocked; insects cannot easily escape by flight from the larger animals
which prey on them; therefore, speaking metaphorically, they are
reduced, like most weak creatures, to trickery and dissimulation.
It should be observed that the process of imitation probably never
commenced between forms widely dissimilar in colour. But starting with
species already somewhat like each other, the closest resemblance,
if beneficial, could readily be gained by the above means; and if
the imitated form was subsequently and gradually modified through
any agency, the imitating form would be led along the same track,
and thus be altered to almost any extent, so that it might
ultimately assume an appearance or colouring wholly unlike that of the
other members of the family to which it belonged. There is, however,
some difficulty on this head, for it is necessary to suppose in some
cases that ancient members belonging to several distinct groups,
before they had diverged to their present extent, accidentally
resembled a member of another and protected group in a sufficient
degree to afford some slight protection; this having given the basis
for the subsequent acquisition of the most perfect resemblance.
On the Nature of the Affinities connecting Organic Beings.- As the
modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages which made the groups to
which they belong large and their parents dominant, they are almost
sure to spread widely, and to seize on more and more places in the
economy of nature. The larger and more dominant groups within each
class thus tend to go on increasing in size; and they consequently
supplant many smaller and feebler groups. Thus we can account for
the fact that all organisms, recent and extinct, are included under
a few great orders, and under still fewer classes. As showing how
few the higher groups are in number, and how widely they are spread
throughout the world, the fact is striking that the discovery of
Australia has not added an insect belonging to a new class; and that
in the vegetable kingdom, as I learn from Dr. Hooker, it has added
only two or three families of small size.
In the chapter on Geological Succession I attempted to show, on
the principle of each group having generally diverged much in
character during the long-continued process of modification, how it is
that the more ancient forms of life often present characters in some
degree intermediate between existing groups. As some few of the old
and intermediate forms have transmitted to the present day descendants
but little modified, these constitute our so-called osculant or
aberrant species. The more aberrant any form is, the greater must be
the number of connecting forms which have been exterminated and
utterly lost. And we have some evidence of aberrant groups having
suffered severely from extinction, for they are almost always
represented by extremely few species; and such species as do occur are
generally very distinct from each other, which again implies
extinction. The genera Ornithorhynchus and lepidosiren, for example,
would not have been less aberrant had each been represented by a
dozen species, instead of as at present by a single one, or by two
or three. We can, I think, account for this fact only by looking at
aberrant groups as forms which have been conquered by more
successful competitors, with a few members still preserved under
unusually favourable conditions.
Mr. Waterhouse has remarked that, when a member belonging to one
group of animals exhibits an affinity to a quite distinct group,
this affinity in most cases is general and not special; thus,
according to Mr. Waterhouse, of all rodents, the bizcacha is most
nearly related to marsupials; but in the points in which it
approaches this order, its relations are general, that is, not to
any one marsupial species more than to another. As these points of
affinity are believed to be real and not merely adaptive, they must be
due in accordance with our view to inheritance from a common
progenitor. Therefore we must suppose either that all rodents,
including the bizcacha, branched off from some ancient marsupial,
which will naturally have been more or less intermediate in
character with respect to all existing marsupials; or that both
rodents and marsupials branched off from a common progenitor, and that
both groups have since undergone much modification in divergent
directions. On either view we must suppose that the bizcacha has
retained, by inheritance, more of the, characters of its ancient
progenitor than have other rodents; and therefore it will not be
specially related to any one existing marsupial, but indirectly to all
or nearly all marsupials, from having partially retained the character
of their common progenitor, or of some early member of the group. On
the other hand, of all marsupials, as Mr. Waterhouse has remarked, the
Phascolomys resembles most nearly, not any one species, but the
general order of rodents. In this case, however, it may be strongly
suspected as the resemblance is only analogical, owing to the
Phascolomys having become adapted to habits like those of a rodent.
The elder De Candolle has made nearly similar observations on the
general nature of the affinities of distinct families of plants.
On the principle of the multiplication and gradual divergence in
character of the species descended from a common progenitor,
together with their retention by inheritance of some characters in
common, we can understand the excessively complex and radiating
affinities by which all the members of the same family or higher group
are connected together. For the common progenitor of a whole family,
now broken up by extinction into distinct groups and sub-groups,
will have transmitted some of its characters, modified in various ways
and degrees, to all the species; and they will consequently be related
to each other by circuitous lines of affinity of various lengths (as
may be seen in the diagram so often referred to), mounting up through
many predecessors. As it is difficult to show the blood relationship
between the numerous kindred of any ancient and noble family even by
the aid of a genealogical tree, and almost impossible to do so without
this aid, we can understand the extraordinary difficulty which
naturalists have experienced in describing, without the aid of a
diagram, the various affinities which they perceive between the many
living and extinct members of the same great natural class.
Extinction, as we have seen in the fourth chapter, has played an
important part in defining and widening the intervals between the
several groups in each class. We may thus account for the distinctness
of whole classes from each other- for instance, of birds from all
other vertebrate animals- by the belief that many ancient forms of
life have been utterly lost, through which the early progenitors of
birds were formerly connected with the early progenitors of the
other and at that time less differentiated vertebrate classes. There
has been much less extinction of the forms of life which once
connected fishes with batrachians. There has been still less within
some whole classes, for instance the Crustacea, for here the most
wonderfully diverse forms are still linked together by a long and only
partially broken chain of affinities. Extinction has only defined
the groups: it has by no means made them; for if every form which
has ever lived on this earth were suddenly to reappear, though it
would be quite impossible to give definitions by which each group
could be distinguished, still a natural classification, or at least
a natural arrangement, would be possible. We shall see this by turning
to the diagram; the letters, A to L, may represent eleven Silurian
genera, some of which have produced large groups of modified
descendants, with every link in each branch and sub-branch still
alive; and the links not greater than those between existing
varieties. In this case it would be quite impossible to give
definitions by which the several members of the several groups could
be distinguished from their more immediate parents and descendants.
Yet the arrangement in the diagram would still hold good and would be
natural; for, on the principle of inheritance, all the forms
descended, for instance, from A, would have something in common. In a
tree we can distinguish this or that branch, though at the actual fork
the two unite and blend together. We could not, as I have said, define
the several groups; but we could pick out types, or forms,
representing most of the characters of each group, whether large or
small, and thus give a general idea of the value of the differences
between them. This is what we should be driven to, if we were ever to
succeed in collecting all the forms in any one class which have lived
throughout all time and space. Assuredly we shall never succeed in
making so perfect a collection: nevertheless, in certain classes, we
are tending towards: this end; and Milne Edwards has lately insisted,
in an able paper, on the high importance of looking to types, whether
or not we can separate and define the groups to which such types
belong.
Finally we have seen that natural selection, which follows from
the struggle for existence, and which almost inevitably leads to
extinction and divergence of character in the descendants from any one
parent species, explains that great and universal feature in the
affinities of all organic beings, namely, their subordination in group
under group. We use the element of descent in classing the individuals
of both sexes and of all ages under one species, although they may
have but few characters in common; we use descent in classing
acknowledged varieties, however different they may be from their
parents; and I believe that this element of descent is the hidden bond
of connection which naturalists have sought under the term of the,
Natural System. On this idea of the natural system, being, in so far
as it has been perfected, genealogical in its arrangement, with the
grades of difference expressed by the terms genera, families,
orders, &c., we can understand the rules which we are compelled to
follow in our classification. We can understand why we value certain
resemblances far more than others; why we use rudimentary and
useless organs, or others of trifling physiological importance; why,
in finding the relations between one group and another, we summarily
reject analogical or adaptive characters, and yet use these same
characters within the limits of the same group. We can clearly see how
it is that all living and extinct forms can be grouped together within
a few great classes; and how the several members of each class are
connected together by the most complex and radiating lines of
affinities. We shall never, probably, disentangle the inextricable web
of the affinities between the members of any one class; but when we
have a distinct object in view, and do not look to some unknown plan
of creation, we may hope to make sure but slow progress.
Professor Haeckel in his Generelle Morphologie and in other works,
has recently brought his great knowledge and abilities to bear on what
he calls phylogeny, or the lines of descent of all organic beings.
In drawing up the several series he trusts chiefly to embryological
characters, but receives aid from homologous and rudimentary organs,
as well as from the successive periods at which the various forms of
life are believed to have first appeared in our geological formations.
He has thus boldly made a great beginning, and shows us how
classification will in the future be treated.
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