CHAPTER VI - DIFFICULTIES OF THE THEORY
Modes of Transition
If it could be demonstrated that any complex organ existed, which
could not possibly have been formed by numerous, successive, slight
modifications, my theory would absolutely break down. But I can find
out no such case. No doubt many organs exist of which we do not know
the transitional grades, more especially if we look to much-isolated
species, round which, according to the theory, there has been much
extinction. Or again, if we take an organ common to all the members of
a class, for in this latter case the organ must have been originally
formed at a remote period, since which all the many members of the
class have been developed; and in order to discover the early
transitional grades through which the organ has passed, we should have
to look to very ancient ancestral forms, long since become extinct.
We should be extremely cautious in concluding that an organ could
not have been formed by transitional gradations of some kind. Numerous
cases could be given amongst the lower animals of the same organ
performing at the same time wholly distinct functions; thus in the
larva of the dragon-fly and in the fish Cobitis the alimentary canal
respires, digests, and excretes. In the Hydra, the animal may be
turned inside out, and the exterior surface will then digest and the
stomach respire. In such cases natural selection might specialise,
if any advantage were thus gained, the whole or part of an organ,
which had previously performed two functions, for one function
alone, and thus by insensible steps greatly change its nature. Many
plants are known which regularly produce at the same time
differently constructed flowers; and if such plants were to produce
one kind alone, a great change would be effected with comparative
suddenness in the character of the species. It is, however, probable
that the two sorts of flowers borne by the same plant were
originally differentiated by finely graduated steps, which may still
be followed in some few cases.
Again, two distinct organs, or the same organ under two very
different forms, may simultaneously perform in the same individual the
same function, and this is an extremely important means of transition:
to give one instance,- there are fish with gills or branchiae that
breathe the air dissolved in the water, at the same time that they
breathe free air in their swimbladders, this latter organ being
divided by highly vascular partitions and having a ductus
pneumaticus for the supply of air. To give another instance from the
vegetable kingdom: plants climb by three distinct means, by spirally
twining, by clasping a support with their sensitive tendrils, and by
the emission of aerial rootlets; these three means are usually found
in distinct groups, but some few species exhibit two of the means,
or even all three, combined in the same individual. In all such
cases one of the two organs might readily be modified and perfected so
as to perform all the work, being aided during the progress of
modification by the other organ; and then this other organ might be
modified for some other and quite distinct purpose, or be wholly
obliterated.
The illustration of the swimbladder in fishes is a good one, because
it shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely, flotation, may be converted
into one for a widely different purpose, namely, respiration. The
swimbladder has, also, been worked in as an accessory to the
auditory organs of certain fishes. All physiologists admit that the
swimbladder is homologous, or "ideally similar" in position and
structure with the lungs of the higher vertebrate animals: hence there
is no reason to doubt that the swimbladder has actually been converted
into lungs, or an organ used exclusively for respiration.
According to this view it may be inferred that all vertebrate
animals with true lungs are descended by ordinary generation from an
ancient and unknown prototype, which was furnished with a floating
apparatus or swimbladder. We can thus, as I infer from Owen's
interesting description of these parts, understand the strange fact
that every particle of food and drink & which we swallow has to pass
over the orifice of the trachea, with some risk of falling into the
lungs, notwithstanding the beautiful contrivance by which the
glottis is closed. In the higher Vertebrate the branchiae have
wholly disappeared- but in the embryo the slits on the sides of the
neck and the loop-like course of the arteries still mark their
former position. But it is conceivable that the now utterly lost
branchiae might have been gradually worked in by natural selection for
some distinct purpose: for instance, Landois has shown that the
wings of insects are developed from the tracheae; it is therefore
highly probable that in this great class organs which once served
for respiration have been actually converted into organs for flight.
In considering transitions of organs, it is so important to bear
in mind the probability of conversion from one function to another,
that I will give another instance. Pedunculated cirripedes have two
minute folds of skin, called by me the ovigerous frena, which serve,
through the means of a sticky secretion, to retain the eggs until they
are hatched within the sack. These cirripedes have no branchiae, the
whole surface of the body and of the sack, together with the small
frena, serving for respiration. The Balanidae or sessile cirripedes,
on the other hand, have no ovigerous frena, the eggs lying loose at
the bottom of the sack, within the well-enclosed shell; but they have,
in the same relative position with the frena, large, much-folded
membranes, which freely communicate with the circulatory lacunae of
the sack and body, and which have been considered by all naturalists
to act as branchiae. Now I think no one will dispute that the
ovigerous frena in the one family are strictly homologous with the
branchiae of the other family; indeed, they graduate into each
other. Therefore it need not be doubted that the two little folds of
skin, which originally served as ovigerous frena, but which, likewise,
very slightly aided in the act of respiration, have been gradually
converted by natural selection into branchiae simply through an
increase in their size and the obliteration of their adhesive
glands. If all pedunculated cirripedes had become extinct, and they
have suffered far more extinction than have sessile cirripedes, who
would ever have imagined that the branchiae in this latter family
had originally existed as organs for preventing the ova from being
washed out of the sack?
There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has
lately been insisted on by Prof. Cope and others in the United States.
It is now known that some animals are capable of reproduction at a
very early age, before they have acquired their perfect characters;
and if this power became thoroughly well developed in a species, it
seems probable that the adult stage of development would sooner or
later be lost; and in this case, especially if the larva differed much
from the mature form, the character of the species would be greatly
changed and degraded. Again, not a few animals, after arriving at
maturity, go on changing in character during nearly their whole lives.
With mammals, for instance, the form of the skull is often much
altered with age, of which Dr. Murie has given some striking instances
with seals; every one knows how the horns of stags become more and
more branched, and the plumes of some birds become more finely
developed, as they grow older. Prof. Cope states that the teeth of
certain lizards change much in shape with advancing years; with
crustaceans not only many trivial, but some important parts assume a
new character, as recorded by Fritz Muller, after maturity. In all
such cases,- and many could be given,- if the age for reproduction
were retarded, the character of the species, at least in its adult
state, would be modified; nor is it improbable that the previous and
earlier stages of development would in some cases be hurried through
and finally lost. Whether species have often or ever been modified
through this comparatively sudden mode of transition, I can form no
opinion; but if this has occurred, it is probable that the differences
between the young and the mature, and between the mature and the
old, were primordially acquired by graduated steps.
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