CHAPTER VII - MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION
I WILL devote this chapter to the consideration of various
miscellaneous objections which have been advanced against my views, as
some of the previous discussions may thus be made clearer; but it
would be useless to discuss all of them, as many have been made by
writers who have not taken the trouble to understand the subject. Thus
a distinguished German naturalist has asserted that the weakest part
of my theory is, that I consider all organic beings as imperfect: what
I have really said is, that all are not as perfect as they might
have been in relation to their conditions; and this is shown to be the
case by so many native forms in many quarters of the world having
yielded their places to intruding foreigners. Nor can organic
beings, even if they were at any one time perfectly adapted to their
conditions of life, have remained so, when their conditions changed,
unless they themselves likewise changed; and no one will dispute
that the physical conditions of each country, as well as the numbers
and kinds of its inhabitants, have undergone many mutations.
A critic has lately insisted, with some parade of mathematical
accuracy, that longevity is a great advantage to all species, so
that he who believes in natural selection "must arrange his
genealogical tree" in such a manner that all the descendants have
longer lives than their progenitors! Cannot our critic conceive that a
biennial plant or one of the lower animals might range into a cold
climate and perish there every winter; and yet, owing to advantages
gained through natural selection, survive from year to year by means
of its seeds or ova? Mr. E. Ray Lankester has recently discussed
this subject, and he concludes, as far as its extreme complexity
allows him to form a judgment, that longevity is generally related
to the standard of each species in the scale of organisation, as
well as to the amount of expenditure in reproduction and in general
activity. And these conditions have, it is probable, been largely
determined through natural selection.
It has been argued that, as none of the animals and plants of Egypt,
of which we know anything, have changed during the last three or
four thousand years, so probably have none in any part of the world.
But, as Mr. G. H. Lewes has remarked, this line of argument proves too
much, for the ancient domestic races figured on the Egyptian
monuments, or embalmed, are closely similar or even identical with
those now living; yet all naturalists admit that such races have
been produced through the modification of their original types. The
many animals which have remained unchanged since the commencement of
the glacial period, would have been an incomparably stronger case, for
these have been exposed to great changes of climate and have
migrated over great distances; whereas, in Egypt, during the last
several thousand years, the conditions of life, as far as we know,
have remained absolutely uniform. The fact of little or no
modification having been effected since the glacial period would
have been of some avail against those who believe in an innate and
necessary law of development, but is powerless against the doctrine of
natural selection or the survival of the fittest, which implies that
when variations or individual differences of a beneficial nature
happen to arise, these will be preserved; but this will be effected
only under certain favourable circumstances.
The celebrated palaeontologist, Bronn, at the close of his German
translation of this work, asks, how, on the principle of natural
selection, can a variety live side by side with the parent species? If
both have become fitted for slightly different habits of life or
conditions, they might live together; and if we lay on one side
polymorphic species, in which the variability seems to be of a
peculiar nature, and all mere temporary variations, such as size,
albinism, &c., the more permanent varieties are generally found, as
far as I can discover, inhabiting distinct stations,- such as high
land or low land, dry or moist districts. Moreover, in the case of
animals which wander much about and cross freely, their varieties seem
to be generally confined to distinct regions.
Bronn also insists that distinct species never differ from each
other in single characters, but in many parts; and he asks, how it
always comes that many parts of the organisation should have been
modified at the same time through variation and natural selection .
" But there is no necessity for supposing that all the parts of any
being have been simultaneously modified. The most striking
modifications, excellently adapted for some purpose, might, as was
formerly remarked, be acquired by successive variations, if slight,
first in one part and then in another; and as they would be
transmitted all together, they would appear to us as if they had
been simultaneously developed. The best answer, however, to the
above objection is afforded by those domestic races which have been
modified, chiefly through man's power of selection, for some special
purpose. Look at the race and dray horse, or at the greyhound and
mastiff. Their whole frames and even their mental characteristics have
been modified; but if we could trace each step in the history of their
transformation,- and the latter steps can be traced,- we should not
see great and simultaneous changes, but first one part and then
another slightly modified and improved. Even when selection has been
applied by man to some one character alone,- of which our cultivated
plants offer the best instances,- it will invariably be found that
although this one part, whether it be the flower, fruit, or leaves,
has been greatly changed, almost all the other parts have been
slightly modified. This may be attributed partly to the principle of
correlated growth, and partly to so-called spontaneous variation.
A much more serious objection has been urged by Bronn, and
recently by Broca, namely, that many characters appear to be of no
service whatever to their possessors, and therefore cannot have been
influenced through natural selection. Bronn adduces the length of
the ears and tails in the different species of hares and mice,- the
complex folds of enamel in the teeth of many animals, and a
multitude of analogous cases. With respect to plants, this subject has
been discussed by Nageli in an admirable essay. He admits that natural
selection has effected much, but he insists that the families of
plants differ chiefly from each other in morphological characters,
which appear to be quite unimportant for the welfare of the species.
He consequently believes in an innate tendency towards progressive and
more perfect development. He specifies the arrangement of the cells in
the tissues, and of the leaves on the axis, as cases in which
natural selection could not have acted. To these may be added the
numerical divisions in the parts of the flower, the position of the
ovules, the shape of the seed, when not of any use for
dissemination, &c.
There is much force in the above objection. Nevertheless, we
ought, in the first place, to be extremely cautious in pretending to
decide what structures now are, or have formerly been, use to each
species. In the second place, it should always be borne in mind that
when part is modified, so will be other parts, through certain dimly
seen causes, such as an increased or diminished flow of nutriment to a
part, mutual pressure, an early developed part affecting one
subsequently developed, and so forth,- as well as through other causes
which lead to the many mysterious cases of correlation, which we do
not in the least understand. These agencies may be all grouped
together, for the sake of brevity, under the expression of the laws of
growth. In the third place, we have to allow for the direct and
definite action of changed conditions of life, and for so-called
spontaneous variations, in which the nature of the conditions
apparently plays a quite subordinate part. Bud-variations, such as the
appearance of a moss-rose on a common rose, or of a nectarine on a
peach tree offer good instances of spontaneous variations; but even in
these cases, if we bear in mind the power of a minute drop of poison
in producing complex galls, we ought not to feel too sure that the
above variations are not the effect of some local change in the nature
of the sap, due to some change in the conditions. There must be some
efficient cause for each slight individual difference, as well as
for more strongly marked variations which occasionally arise; and if
the unknown cause were to act persistently, it is almost certain
that all the individuals of the species would be similarly modified.
In the earlier editions of this work I underrated, as it now seems
probable, the frequency and importance of modifications due to
spontaneous variability. But it is impossible to attribute to this
cause the innumerable structures which are so well adapted to the
habits of life of each species. I can no more believe in this than
that the well-adapted form of a race-horse or greyhound, which
before the principle of selection by man was well understood,
excited so much surprise in the minds of the older naturalists, can
thus be explained.
It may be worth while to illustrate some of the foregoing remarks.
With respect to the assumed inutility of various parts and organs,
it is hardly necessary to observe that even in the higher and
best-known animals many structures exist, which are so highly
developed that no one doubts that they are of importance, yet their
use has not been, or has only recently been, ascertained. As Bronn
gives the length of the ears and tail in the several species of mice
as instances, though trifling ones, of differences in structure
which can be of no special use, I may mention that, according to Dr.
Schobl, the external ears of the common mouse are supplied in an
extraordinary manner with nerves, so that they no doubt serve as
tactile organs; hence the length of the ears can hardly be quite
unimportant. We shall, also, presently see that the tail is a highly
useful prehensile organ to some of the species; and its use would be
much influenced by its length.
With respect to plants, to which on account of Nageli's essay I
shall confine myself in the following remarks, it will be admitted
that the flowers of orchids present a multitude of curious structures,
which a few years ago would have been considered as mere morphological
differences without any special function; but they are now known to be
of the highest importance for the fertilisation of the species through
the aid of insects, and have probably been gained through natural
selection. No one until lately would have imagined that in dimorphic
and trimorphic plants the different lengths of the stamens and
pistils, and their arrangement, could have been of any service, but
now we know this to be the case.
In certain whole groups of plants the ovules stand erect, and in
others they are suspended; and within the same ovarium of some few
plants, one ovule holds the former and a second ovule the latter
position. These positions seem at first purely morphological, or of no
physiological signification; but Dr. Hooker informs me that within the
same ovarium, the upper ovules alone in some cases, and in other cases
the lower ones alone are fertilised; and he suggests that this
probably depends on the direction in which the pollen-tubes enter
the ovarium. If so, the position of the ovules, even when one is erect
and the other suspended within the same ovarium, would follow from the
selection of any slight deviations in position which favoured their
fertilisation, and the production of seed.
Several plants belonging to distinct orders habitually produce
flowers of two kinds,- the one open of the ordinary structure, the
other closed and imperfect. These two kinds of flowers sometimes
differ wonderfully in structure, yet may be seen to graduate into each
other on the same plant. The ordinary and open flowers can be
intercrossed; and the benefits which certainly are derived from this
process are thus secured. The closed and imperfect flowers are,
however, manifestly of high importance, as they yield with the
utmost safety a large stock of seed, with the expenditure of
wonderfully little pollen. The two kinds of flowers often differ much,
as just stated, in structure. The petals in the imperfect flowers
almost always consist of mere rudiments, and the pollen-grains are
reduced in diameter. In Ononis columnae five of the alternate
stamens are rudimentary; and in some species of Viola three stamens
are in this state, two retaining their proper function, but being of
very small size. In six out of thirty of the closed flowers in an
Indian violet (name unknown, for the plants have never produced with
me perfect flowers), the sepals are reduced from the normal number
of five to three. In one section of the Malpighiaceae the closed
flowers, according to A. de Jussieu, are still further modified, for
the five stamens which stand opposite to the sepals are all aborted,
sixth stamen standing opposite to a petal being alone developed; and
this stamen is not present in the ordinary flowers of these species;
the style is aborted; and the ovaria are reduced from three to two.
Now although natural selection may well have had the power to
prevent some of the flowers from expanding, and to reduce the amount
of pollen, when rendered by the closure of the flowers superfluous,
yet hardly any of the above special modifications can have been thus
determined, but must have followed from the laws of growth,
including the functional inactivity of parts, during the progress of
the reduction of the pollen and the closure of the flowers.
It is so necessary to appreciate the important effects of the laws
of growth, that I will give some additional cases of another kind,
namely of differences in the same part or organ, due to differences in
relative position on the same plant. In the Spanish chestnut, and in
certain fir-trees, the angles of divergence of the leaves differ,
according to Schacht, in the nearly horizontal and in the upright
branches. In the common rue and some other plants, one flower, usually
the central or terminal one, opens first, and has five sepals and
petals, and five divisions to the ovarium; whilst all the other
flowers on the plant are tetramerous. In the British Adoxa the
uppermost flower generally has two calyx-lobes with the other organs
tetramerous, whilst the surrounding flowers generally have three
calyx-lobes with the other organs pentamerous. In many Compositae
and Umbelliferae (and in some other plants) the circumferential
flowers have their corollas much more developed than those of the
centre; and this seems often connected with the abortion of the
reproductive organs. It is a more curious fact, previously referred
to, that the achenes or seeds of the circumference and centre
sometimes differ greatly in form, colour, and other characters. In
Carthamus and some other Compositae the central achenes alone are
furnished with a pappus; and in Hyoseris the same head yields
achenes of three different forms. In certain Umbelliferae the exterior
seeds, according to Tausch, are orthospermous, and the central one
coelospermous, and this is a character which was considered by De
Candolle to be in other species of the highest systematic
importance. Prof. Braun mentions a Fumariaceous genus, in which the
flowers in the lower part of the spike bear oval, ribbed, one-seeded
nutlets; and in the upper part of the spike, lanceolate, two-valved,
and two-seeded siliques. In these several cases, with the exception of
that of the well developed rayflorets, which are of service in
making the flowers conspicuous to insects, natural selection cannot,
as far as we can judge, have come into play, or only in a quite
subordinate manner. All these modifications follow from the relative
position and inter-action of the parts; and it can hardly be doubted
that if all the flowers and leaves on the same plant had been
subjected to the same external and internal condition, as are the
flowers and leaves in certain positions, all would have been
modified in the same manner.
In numerous other cases we find modifications of structure, which
are considered by botanists to be generally of a highly important
nature, affecting only some of the flowers on the same plant, or
occurring on distinct plants, which grow close together under the same
conditions. As these variations seem of no special use to the
plants, they cannot have been influenced by natural selection. Of
their cause we are quite ignorant; we cannot even attribute them, as
in the last class of cases, to any proximate agency, such as
relative position. I will give only a few instances. It is so common
to observe on the same plant, flowers indifferently tetramerous,
pentamerous, &c., that I need not give examples; but as numerical
variations are comparatively rare when the parts are few, I may
mention that, according to De Candolle, the flowers of Papaver
bracteatum offer either two sepals with four petals (which is the
common type with poppies), or three sepals with six petals. The manner
in which the petals are folded in the bud is in most groups a very
constant morphological character; but Professor Asa Gray states that
with some species of Mimulus, the aestivation is almost as
frequently that of the Rhinanthideae as of the Antirrhinideae, to
which latter tribe the genus belongs. Auguste de Saint-Hilaire gives
the following cases: the genus Zanthoxylon belongs to a division of
the Rutacese with a single ovary, but in some species flowers may be
found on the same plant, and even in the same panicle, with either one
or two ovaries. In Helianthemum the capsule has been described as
unilocular or trilocular; and in H. mutabile, "Une lame, plus ou moins
large, s'etend entre le pericarpe et le placenta." In the flowers of
Saponaria officinalis, Dr. Masters has observed instances of both
marginal and free central placentation. Lastly, Saint-Hilaire found
towards the southern extreme of the range of Gomphia oleaeformis two
forms which he did not at first doubt were distinct species, but he
subsequently saw them growing on the same bush; and he then adds,
"Voila donc dans un meme individu des loges et un style qui se
rattachent tantot a un axe verticale et tantot a un gynobase."
We thus see that with plants many morphological changes may be
attributed to the laws of growth and the inter-action of parts,
independently of natural selection. But with respect to Nageli's
doctrine of an innate tendency towards perfection or progressive
development, can it be said in the case of these strongly pronounced
variations, that the plants have been caught in the act of progressing
towards a higher state of development? On the contrary, I should infer
from the mere fact of the parts in question differing or varying
greatly on the same plant, that such modifications were of extremely
small importance to the plants themselves, of whatever importance they
may generally be to us for our classifications. The acquisition of a
useless part can hardly be said to raise an organism in the natural
scale; and in the case of the imperfect, closed flowers above
described, if any new principle has to be invoked, it must be one of
retrogression rather than of progression; and so it must be with
many parasitic and degraded animals. We are ignorant of the exciting
cause of the above specified modifications; but if the unknown cause
were to act almost uniformly for a length of time, we may infer that
the result would be almost uniform; and in this case all the
individuals of the species would be modified in the same manner.
From the fact of the above characters being unimportant for the
welfare of the species, any slight variations which occurred in them
would not have been accumulated and augmented through natural
selection. A structure which has been developed through long-continued
selection, when it ceases to be of service to a species, generally
becomes variable, as we see with rudimentary organs; for it will no
longer be regulated by this same power of selection. But when, from
the nature of the organism and of the conditions, modifications have
been induced which are unimportant for the welfare of the species,
they may be, and apparently often have been, transmitted in nearly the
same state to numerous, otherwise modified, descendants. It cannot
have been of much importance to the greater number of mammals,
birds, or reptiles, whether they were clothed with hair, feathers,
or scales; yet hair has been transmitted to almost all mammals,
feathers to all birds, and scales to all true reptiles. A structure,
whatever it may be, which is common to many allied forms, is ranked by
us as of high systematic importance, and consequently is often assumed
to be of high vital importance to the species. Thus, as I am
inclined to believe, differences, which we consider as important- such
as the arrangement of the leaves, the divisions of the flower or of
the ovarium, the position of the ovules, &c.- first appeared in many
cases as fluctuating variations, which sooner or later became constant
through the nature of the organism and of the surrounding
conditions, as well as through the intercrossing of distinct
individuals, but not through natural selection; for as these
morphological characters do not affect the welfare of the species, any
slight deviations in them could not have been governed or
accumulated through this latter agency. It is a strange result which
we thus arrive at, namely that characters of slight vital importance
to the species, are the most important to the systematist; but, as
we shall hereafter see when we treat of the genetic principle of
classification, this is by no means so paradoxical as it may at
first appear.
Although we have no good evidence of the existence in organic beings
of an innate tendency towards progressive development, yet this
necessarily follows, as I have attempted to show in the fourth
chapter, through the continued action of natural selection. For the
best definition which has ever been given of a high standard of
organisation, is the degree to which the parts have been specialised
or differentiated; and natural selection tends towards this end,
inasmuch as the parts are thus enabled to perform their functions more
efficiently.
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