Charles Darwin.

The origin of species online

. (page 44 of 50)
Online LibraryCharles DarwinThe origin of species → online text (page 44 of 50)
Font size
QR-code for this ebook

parthenogenetic reproduction by gradual steps to an earlier and earlier age
— -Chironomus showing us an almost exactly intermediate stage, viz., that
of the pupa — and we can perhaps account for the marvellous case of the

It has already been stated that various parts in the same individual, which
are exactly alike during an early embryonic period, become widely different
and serve for widely different purposes in the adult state. So again it has
been shown that generally the embryos of the most distinct species belonging
to the same class are closely similar, but become, when fully developed,
widely dissimilar. A better proof of this latter fact cannot be given than the
statement by Von Baer that "the embryos of mammalia, of birds, lizards and
snakes, probably also of chelonia, are in the earliest states exceedingly like
one another, both as a whole and in the mode of development of their parts;
so much so, in fact, that we can often distinguish the embryos only by their
size. In my possession are two little embryos in spirit, whose names I have
omitted to attach, and at present I am quite unable to say to what class they
belong. They may be lizards or small birds, or very young mammalia, so
complete is the similarity in the mode of formation of the head and trunk in
these animals. The extremities, however, are still absent in these embryos.
But even if they had existed in the earliest stage of their development we
should learn nothing, for the feet of lizards and mammals, the wings and feet
of birds, no less than the hands and feet of man, all arise from the same
fundamental form." The larvae of most crustaceans, at corresponding stages
of development, closely resemble each other, however different the adults
may become; and so it is with very many other animals. A trace of the law
of embryonic resemblance occasionally lasts till a rather late age : thus birds
of the same genus, and of allied genera, often resemble each other in their
immature plumage; as we see in the spotted feathers in the young of the
thrush group. In the cat tribe, most of the species when adult are striped
or spotted in lines; and stripes or spots can be plainly distinguished in the
whelp of the lion and the puma. We occasionally, though rarely, see some-
thing of the same kind in plants; thus the first leaves of the ulex or furze,
and the first leaves of the phyllodineous acacias, are pinnate or divided, like
the ordinary leaves of the leguminosse.

The points of structure, in which the embryos of widely different animals
within the same class resemble each other, often have no direct relation to
their conditions of existence. We cannot, for instance, suppose that in the


embryos of the vertebrata the peculiar, loop-like courses of the arteries near
the branchial slits are related to similar conditions — in the young mammal
which is nourished in the womb of its mother, in the egg of the bird which
is hatched in a nest, and in the spawn of a frog under water. We have no
more reason to believe in such a relation than we have to believe that the
similar bones in the hand of a man, wing of a bat, and fin of a porpoise, are
related to similar conditions of life. No one supposes that the stripes on the
whelp of a Hon, or the spots on the young blackbird, are of any use to these

The case, however, is different when an animal, during any part of its
embryonic career, is active, and has to provide for itself. The period of
activity may come on earlier or later in life; but whenever it comes on, the
adaptation of the larva to its conditions of life is just as perfect and as beauti-
ful as in the adult animal. In how important a manner this has acted, has
recently been well shown by Sir J. Lubbock in his remarks on the close
similarity of the larvae of some insects belonging to very different orders, and
on the dissimilarity of the larvae of other insects within the same order,
according to their habits of life. Owing to such adaptations the similarity
of the larvae of allied animals is sometimes greatly obscured; especially when
there is a division of labor during the different stages of development, as
when the same larva has during one stage to search for food, and during
another stage has to search for a place of attachment. Cases can even be
given of the larvae of allied species, or groups of species, differing more from
each other than do the adults. In most cases, however, the larvae, though
active, still obey, more or less closely, the law of common embryonic re-
semblance. Cirripedes afford a good instance of this; even the illustrious
Cuvier did not percTeive that a barnacle was a crustacean; but a glance at
the larva shows this in an unmistakable manner. So again the two main
divisions of cirripedes, the pedunculated and sessile, though differing widely
in external appearance, have larvae in all their stages barely distinguishable.

The embryo in the course of development generally rises in organization.
I use this expression, though I am aware that it is hardly possible to define
clearly what is meant by organization being higher or lower. But no one
probably will dispute that the butterfly is higher than the caterpillar. In some
cases, however, the mature animal must be considered as lower in the scale
than the larva, as with certain parasitic crustaceans. To refer once again
to cirripedes: the larvae in the first stage have three pairs of locomotive
organs, a simple single eye, and a probosciformed mouth, with which they
feed largely, for they increase much in size. In the second stage, answering
to the chrysahs stage of butterflies, they have six pairs of beautifully con-
structed natatory legs, a pair of magnificent compound eyes, and extremely
complex antennae; but they have a closed and imperfect mouth and cannot
feed: their function at this stage is, to search out by their well-developed
organs of sense, and to reach by their active powers of swimming, a proper
place on which to become attached and to undergo their final metamorphosis.
When this is completed they are fixed for life : their legs are now converted


into prehensile organs; they again obtain a well-constructed mouth; but they
have no antenna, and their two eyes are now reconverted into a minute,
single, simple eye-spot. In this last and complete state, cirripedes may be
considered as either more highly or more lowly organized than they were in
the larval condition. But in some genera the larvae become developed into
hermaphrodites having the ordinary structure, and into what I have called
complemental males; and in the latter the development has assuredly been
retrograde, for the male is a mere sac, which lives for a short time and is
destitute of mouth, stomach, and every other organ of importance, excepting
those for reproduction.

We are so much accustomed to see a difference in structure between the
embryo and the adult, that we are tempted to look at this difference as in
some necessary manner contingent on growth. But there is no reason why,
for instance, the wing of a bat, or the fin of a porpoise, should not have been
sketched out with all their parts in proper proportion, as soon as any part
became visible. In some whole groups of animals and in certain members of
other groups this is the case, and the embryo does not at any period differ
widely from the adult: thus Owen has remarked, in regard to cuttle-fish,
"there is no metamorphosis; the cephalopodic character is manifested long
before the parts of the embryo are completed." Land-shells and fresh-water
crustaceans are born having their proper forms, while the marine members of
the same two great classes pass through considerable and often great changes
during their development. Spiders, again, barely undergo any metamorphosis.
The larvae of most insects pass through a wormlike stage, whether they are
active and adapted to diversified habits, or are inactive from being placed in
the midst of proper nutriment, or from being fed by their parents; but in
some few cases, as in that of Aphis, if we look to the admirable drawings of
the development of this insect, by Professor Huxley, we see hardly any trace
of the vermiform stage.

Sometimes it is only the earlier developmental stages which fail. Thus,
Fritz Miiller has made the remarkable discovery that certain shrimp-like
crustaceans (allied to Penceus) first appear under the simple nauplius-form,
and after passing through two or more zoea-stages, and then through the
mysis-stage, finally acquire their mature structure: now in the whole great
malacostracan order, to which these crustaceans belong, no other member
is as yet known to be first developed under the nauplius-form, though many
appear as zoeas; nevertheless Miiller assigns reasons for his belief, that if
there had been no suppression of development, all these crustaceans would
have appeared as nauplii.

How, then, can we explain these several facts in embryology — namely,
the very general, though not universal, difference in structure between the
embryo and the adult; the various parts in the same individual embryo,
which ultimately become very unlike, and serve for diverse purposes, being
at an early period of growth alike; the common, but not invariable, re-
semblance between the embryos or larvae of the most distinct species in the
same class; the embryo often retaining, while within the egg or womb,


structures which are of no service to it, either at that or at a later period
of life ; on the other hand, larvae which have to provide for their own wants,
being perfectly adapted to the surrounding conditions ; and lastly, the fact of
certain larvae standing higher in the scale of organization than the mature
animal into which they are developed? I believe that all these facts can be
explained as follows.

It is commonly assumed, perhaps from monstrosities affecting the embryo
at a very early period, that slight variations or individual differences neces-
sarily appear at an equally early period. We have little evidence on this head,
but what we have certainly points the other way; for it is notorious that
breeders of cattle, horses, and various fancy animals, cannot positively tell,
until some time after birth, what will be the merits and demerits of their
young animals. We see this plainly in our own children; we cannot tell
whether a child will be tall or short, or what its precise features will be.
The question is not, at what period of life each variation may have been
caused, but at what period the effects are displayed. The cause may have
acted, and I believe often has acted, on one or both parents before the
act of generation. It deserves notice that it is of no importance to a very
young animal, as long as it remains in its mother's womb or in the egg, or
as long as it is nourished and protected by its parent, whether most of its
characters are acquired a little earlier or later in life. It would not signify,
for instance, to a bird which obtained its food by having a much-curved
beak, whether or not while young it possessed a beak of this shape, as long
as it was fed by its parents.

I have stated in the first chapter, that at whatever age a variation first
appears in the parent, it tends to reappear at a corresponding age in the
offspring. Certain variations can only appear at corresponding ages; for in-
stance, peculiarities in the caterpillar, cocoon, or imago states of the silk-
moth; or, again, in the full-grown horns of cattle. But variations which, for
all that we can see, might have first appeared either earlier or later in life,
likewise tend to reappear at a corresponding age in the offspring and parent.
I am far from meaning that this is invariably the case, and I could give
several exceptional cases of variations (taking the word in the largest sense)
which have supervened at an earlier age in the child than in the parent.

These two principles, namely, that slight variations generally appear
at a not very early period of life, and are inherited at a corresponding not
early period, explain, as I believe, all the above specified leading facts in
embryology. But first let us look to a few analogous cases in our domestic
varieties. Some authors who have written on dogs maintain that the grey-
hound and bull-dog, though so different, are really closely allied varieties,
descended from the same wild stock, hence I was curious to see how far
their puppies differed from each other. I was told by breeders that they
differed just as much as their parents, and this, judging by the eye, seemed
almost to be the case; but on actually measuring the old dogs and their
six-days-old puppies, I found that the puppies had not acquired nearly their
full amount of proportional difference. So, again, I was told that the foals


of cart and race horses — breeds which have been almost wholly formed
by selection under domestication — differed as much as the full-grown ani-
mals; but having had careful measurements made of the dams and of three-
days-old colts of race and heavy cart horses, I find that this is by no means
the case.

As we have conclusive evidence that the breeds of the pigeon are de-
scended from a single wild species, I compared the young within twelve
hours after being hatched. I carefully measured the proportions (but will
not here give the details) of the beak, width of mouth, length of nostril and
of eyelid, size of feet and length of leg, in the wild parent species, in pouters,
fantails, runts, barbs, dragons, carriers, and tumblers. Now, some of these
birds, when mature, differ in so extraordinary a manner in the length and
form of beak, and in other characters, that they would certainly have been
ranked as distinct genera if found in a state of nature. But when the nestling
birds of these several breeds were placed in a row, though most of them
could just be distinguished, the proportional differences in the above specified
points were incomparably less than in the full-grown birds. Some character-
istic points of difference — for instance, that of the width of mouth — could ,
hardly be detected in the young. But there was one remarkable exception
to this rule, for the young of the short-faced tumbler differed from the young
of the wild rock-pigeon, and of the other breeds, in almost exacdy the same ]
proportions as in the adult stage. i

These facts are explained by the above two principles. Fanciers select I
their dogs, horses, pigeons, etc., for breeding, when nearly grown up. They
are indifferent whether the desired qualities are acquired earlier or later in |
life, if the full-grown animal possesses them. And the cases just given, more i
especially that of the pigeons, show that the characteristic differences which
have been accumulated by man's selection, and which give value to his
breeds, do not generally appear at a very early period of life, and are in-
herited at a corresponding not early period. But the case of the short-faced
tumbler, which when twelve hours old possessed its proper characters, proves
that this is not the universal rule; for here the characteristic differences
must either have appeared at an earlier period than usual, or, if not so,
the differences must have been inherited, not at a corresponding, but at
an earlier, age.

Now, let us apply these two principles to species in a state of nature.
Let us take a group of birds, descended from some ancient form and modi-
fied through natural selection for different habits. Then, from the many
slight successive variations having supervened in the several species at a
not early age, and having been inherited at a corresponding age, the young
will have been but little modified, and they will still resemble each other
much more closely than do the adults, just as we have seen with the breeds
of the pigeon. We may extend this view to widely distinct structures and
to whole classes. The fore limbs, for instance, which once served as legs
to a remote progenitor, may have become, through a long course of modi-
fication, adapted in one descendant to act as hands, in another as paddles,


in another as wings; but on the above two principles the fore limbs will not
have been much modified in the embryos of these several forms; although
in each form the fore limb will differ greatly in the adult state. Whatever
influence long continued use or disuse may have had in modifying the limbs
or other parts of any species, this will chiefly or solely have affected it when
nearly mature, when it was compelled to use its full powers to gain its own
living; and the effects thus produced will have been transmitted to the
offspring at a corresponding nearly mature age. Thus the young will not
be modified, or will be modified only in a slight degree, through the effects
of the increased use or disuse of parts.

With some animals the successive variations may have supervened at a
very early period of life, or the steps may have been inherited at an earlier
age than that at which they first occurred. In either of these cases, the young
or embryo will closely resemble the mature parent-form, as we have seen
with the short-faced tumbler. And this is the rule of development in certain
whole groups, or in certain sub-groups alone, as with cuttle-fish, land-shells,
fresh-water crustaceans, spiders, and some members of the great class of
insects. With respect to the final cause of the young in such groups not pass-
ing through any metamorphosis, we can see that this would follow from the
following contingencies: namely, from the young having to provide at a
very early age for their own wants, and from their following the same habits
of life with their parents; for in this case it would be indispensable for their
existence that they should be modified in the same manner as their parents.
Again, with respect to the singular fact that many terrestrial and fresh-water
animals do not undergo any metamorphosis, while marine members of the
same groups pass through various transformations, Fritz Miiller has suggested
that the process of slowly modifying and adapting an animal to live on the
land or in fresh water, instead of in the sea, would be greatly simplified by
its not passing through any larval stage; for it is not probable that places
well adapted for both the larval and mature stages, under such new and
greatly changed habits of life, would commonly be found unoccupied or ill-
occupied by other organisms. In this case the gradual acquirement at an
earlier and earlier age of the adult structure would be favored by natural
selection; and all traces of former metamorphoses would finally be lost.

If, on the other hand, it profited the young of an animal to follow habits
of life slightly different from those of the parent-form, and consequently
to be constructed on a slightly different plan, or if it profited a larva already
different from its parent to change still further, then, on the principle of in-
heritance at corresponding ages, the young or the larvae might be rendered
by natural selection more and more different from their parents to any con-
ceivable extent. Differences in the larva might, also, become correlated
with successive stages of its development; so that the larva, in the first stage,
might come to differ greatly from the larva in the second stage, as is the case
with many animals. The adult might also become fitted for sites or habits, in
which organs of locomotion or of the senses, etc., would be useless; and in
this case the metamorphosis would be retrograde.


From the remarks just made we can see how by changes of structure in
the young, in conformity with changed habits of hfe, together with inheri-
tance at corresponding ages, animals might come to pass through stages
of development, perfectly distinct from the primordial condition of their
adult progenitors. Most of our best authorities are now convinced that the
various larval and pupal stages of insects have thus been acquired through
adaptation, and not through inheritance from some ancient form. The curi-
ous case of Sitaris — a beetle which passes through certain unusual stages of
development — ^will illustrate how this might occur. The first larval form is
described by M. Fabre, as an active, minute insect, furnished with six legs,
two long antennae, and four eyes. These larvae are hatched in the nests of
bees; and when the male bees emerge from their burrows, in the spring,
which they do before the females, the larvae spring on them, and afterward
crawl on to the females while paired with the males. As soon as the female
bee deposits her eggs on the surface of the honey stored in the cells, the
larvae of the Sitaris leap on the eggs and devour them. Afterward they
undergo a complete change; their eyes disappear; their legs and antennae
become rudimentary, and they feed on honey; so that they now more closely
resemble the ordinary larvae of insects; ultimately they undergo a further
transformation, and finally emerge as the perfect beetle. Now, if an insect,
undergoing transformations like those of the Sitaris, were to become the
progenitor of a whole new class of insects, the course of development of the
new class would be widely different from that of our existing insects; and
the first larval stage certainly would not represent the former condition of
any adult and ancient form.

On the other hand, it is highly probable that with many animals the
embryonic or larval stages show us, more or less completely, the condition
of the progenitor of the whole group in its adult state. In the great class
of the Crustacea, forms wonderfully distinct from each other, namely, suc-
torial parasites, cirripedes, entomostraca, and even the malacostraca, appear
at first as larvae under the nauplius form; and as these larvae live, and feed in
the open sea, and are not adapted for any peculiar habits of life, and from
other reasons assigned by Fritz Miiller, it is probable that at some very re-
mote period an independent adult animal, resembling the Nauplius, existed,
and subsequently produced, along several divergent lines of descent, the
above-named great Crustacean groups. So again, it is probable, from what
we knov/ of the embryos of mammals, birds, fishes, and reptiles, that these
animals are the modified descendants of some ancient progenitor, which
was furnished in its adult state with branchiae, a swim-bladder, four fin-Hke
limbs, and a long tail, all fitted for an aquatic fife.

As all the organic beings, extinct and recent, which have ever lived, can
be arranged within a few great classes; and as all within each class have,
according to our theory, been connected together by fine gradations, the
best, and, if our collections were nearly perfect, the only possible arrange-
ment would be genealogical; descent being the hidden bond of connection
which naturalists have been seeking under the term of the Natural System.


On this view we can understand how it is that, in the eyes of most naturaHsts,
the structure of the embryo is even more important for classification than
that of the adult. In two or more groups of animals, however much they
may differ from each other in structure and habits in their adult condition,
if they pass through closely similar embryonic stages, we may feel assured
that they all are descended from one parent-form, and are therefore closely
related. Thus, community in embryonic structure reveals community of
descent; but dissimilarity in embryonic development does not prove dis-
community of descent, for in one of two groups the developmental stages
may have been suppressed, or may have been so greatly modified through
adaptation to new habits of life as to be no longer recognizable. Even in
groups in which the adults have been modified to an extreme degree, com-
munity of origin is often revealed by the structure of the larvae; we have
seen, for instance, that cirripedes, though externally so like shell-fish, are
at once known by their larvae to belong to the great class of crustaceans. As
the embryo often shows us more or less plainly the structure of the less
modified and ancient progenitor of the group, we can see why ancient and
extinct forms so often resemble in their adult state the embr^^os of existing
species of the same class. Agassiz believes this to be a universal law of
nature; and we may hope hereafter to see the law proved true. It can, how-

Online LibraryCharles DarwinThe origin of species → online text (page 44 of 50)