of cattle, horses, and various fancy animals, cannot positively tell, until
some time after the animal has been born, what its merits or form will
ultimately turn out. We see this plainly in our own children; we cannot
always tell whether the child will be tall or short, or what its precise
features will be. The question is not, at what period of life any
variation has been caused, but at what period it is fully displayed. The
cause may have acted, and I believe generally has acted, even before the
embryo is formed; and the variation may be due to the male and female
sexual elements having been affected by the conditions to which either
parent, or their ancestors, have been exposed. Nevertheless an effect thus
caused at a very early period, even before the formation of the embryo, may
appear late in life; as when an hereditary disease, which appears in old
age alone, has been communicated to the offspring from the reproductive
element of one parent. Or again, as when the horns of cross-bred cattle
have been affected by the shape of the horns of either parent. For the
welfare of 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,
it must be quite unimportant whether most of its characters are fully
acquired a little earlier or later in life. It would not signify, for
instance, to a bird which obtained its food best by having a long beak,
whether or not it assumed a beak of this particular length, as long as it
was fed by its parents. Hence, I conclude, that it is quite possible, that
each of the many successive modifications, by which each species has
acquired its present structure, may have supervened at a not very early
period of life; and some direct evidence from our domestic animals supports
this view. But in other cases it is quite possible that each successive
modification, or most of them, may have appeared at an extremely early
period.
I have stated in the first chapter, that there is some evidence to render
it probable, that at whatever age any 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 instance,
peculiarities in the caterpillar, cocoon, or imago states of the silk-moth;
or, again, in the horns of almost full-grown cattle. But further than
this, variations which, for all that we can see, might have appeared
earlier or later in life, tend to appear at a corresponding age in the
offspring and parent. I am far from meaning that this is invariably the
case; and I could give a good many 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, if their truth be admitted, will, I believe, explain
all the above specified leading facts in embryology. But first let us look
at a few analogous cases in domestic varieties. Some authors who have
written on Dogs, maintain that the greyhound and bulldog, though appearing
so different, are really varieties most closely allied, and have probably
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 nearly
acquired their full amount of proportional difference. So, again, I was
told that the foals of cart and race-horses differed as much as the
full-grown animals; and this surprised me greatly, as I think it probable
that the difference between these two breeds has been wholly caused by
selection under domestication; but having had careful measurements made of
the dam and of a three-days old colt of a race and heavy cart-horse, I find
that the colts have by no means acquired their full amount of proportional
difference.
As the evidence appears to me conclusive, that the several domestic breeds
of Pigeon have descended from one wild species, I compared young pigeons of
various breeds, within twelve hours after being hatched; I carefully
measured the proportions (but will not here give details) of the beak,
width of mouth, length of nostril and of eyelid, size of feet and length of
leg, in the wild stock, in pouters, fantails, runts, barbs, dragons,
carriers, and tumblers. Now some of these birds, when mature, differ so
extraordinarily in length and form of beak, that they would, I cannot
doubt, be ranked in distinct genera, had they been natural productions.
But when the nestling birds of these several breeds were placed in a row,
though most of them could be distinguished from each other, yet their
proportional differences in the above specified several points were
incomparably less than in the full-grown birds. Some characteristic 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 all its proportions,
almost exactly as much as in the adult state.
The two principles above given seem to me to explain these facts in regard
to the later embryonic stages of our domestic varieties. Fanciers select
their horses, dogs, and pigeons, for breeding, when they are nearly grown
up: they are indifferent whether the desired qualities and structures have
been acquired earlier or later in life, if the full-grown animal possesses
them. And the cases just given, more especially that of pigeons, seem to
show that the characteristic differences which give value to each breed,
and which have been accumulated by man's selection, have not generally
first appeared at an early period of life, and have been inherited by the
offspring at a corresponding not early period. But the case of the
short-faced tumbler, which when twelve hours old had acquired its proper
proportions, 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
the corresponding, but at an earlier age.
Now let us apply these facts and the above two principles--which latter,
though not proved true, can be shown to be in some degree probable--to
species in a state of nature. Let us take a genus of birds, descended on
my theory from some one parent-species, and of which the several new
species have become modified through natural selection in accordance with
their diverse habits. Then, from the many slight successive steps of
variation having supervened at a rather late age, and having been inherited
at a corresponding age, the young of the new species of our supposed genus
will manifestly tend to resemble each other much more closely than do the
adults, just as we have seen in the case of pigeons. We may exte
nd this
view to whole families or even classes. The fore-limbs, for instance,
which served as legs in the parent-species, may become, by a long course of
modification, adapted in one descendant to act as hands, in another as
paddles, in another as wings; and on the above two principles--namely of
each successive modification supervening at a rather late age, and being
inherited at a corresponding late age--the fore-limbs in the embryos of the
several descendants of the parent-species will still resemble each other
closely, for they will not have been modified. But in each individual new
species, the embryonic fore-limbs will differ greatly from the fore-limbs
in the mature animal; the limbs in the latter having undergone much
modification at a rather late period of life, and having thus been
converted into hands, or paddles, or wings. Whatever influence
long-continued exercise or use on the one hand, and disuse on the other,
may have in modifying an organ, such influence will mainly affect the
mature animal, which has come to its full powers of activity and has to
gain its own living; and the effects thus produced will be inherited at a
corresponding mature age. Whereas the young will remain unmodified, or be
modified in a lesser degree, by the effects of use and disuse.
In certain cases the successive steps of variation might supervene, from
causes of which we are wholly ignorant, at a very early period of life, or
each step might be inherited at an earlier period than that at which it
first appeared. In either case (as with the short-faced tumbler) the young
or embryo would closely resemble the mature parent-form. We have seen that
this is the rule of development in certain whole groups of animals, as with
cuttle-fish and spiders, and with a few members of the great class of
insects, as with Aphis. With respect to the final cause of the young in
these cases not undergoing any metamorphosis, or closely resembling their
parents from their earliest age, we can see that this would result from the
two following contingencies; firstly, from the young, during a course of
modification carried on for many generations, having to provide for their
own wants at a very early stage of development, and secondly, from their
following exactly the same habits of life with their parents; for in this
case, it would be indispensable for the existence of the species, that the
child should be modified at a very early age in the same manner with its
parents, in accordance with their similar habits. Some further
explanation, however, of the embryo not undergoing any metamorphosis is
perhaps requisite. If, on the other hand, it profited the young to follow
habits of life in any degree different from those of their parent, and
consequently to be constructed in a slightly different manner, then, on the
principle of inheritance at corresponding ages, the active young or larvae
might easily be rendered by natural selection different to any conceivable
extent from their parents. Such differences might, also, become correlated
with successive stages of development; so that the larvae, in the first
stage, might differ greatly from the larvae in the second stage, as we have
seen to be the case with cirripedes. The adult might become fitted for
sites or habits, in which organs of locomotion or of the senses, &c., would
be useless; and in this case the final metamorphosis would be said to be
retrograde.
As all the organic beings, extinct and recent, which have ever lived on
this earth have to be classed together, and as all have been connected by
the finest gradations, the best, or indeed, if our collections were nearly
perfect, the only possible arrangement, would be genealogical. Descent
being on my view the hidden bond of connexion 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 naturalists, the structure
of the embryo is even more important for classification than that of the
adult. For the embryo is the animal in its less modified state; and in so
far it reveals the structure of its progenitor. In two groups of animal,
however much they may at present differ from each other in structure and
habits, if they pass through the same or similar embryonic stages, we may
feel assured that they have both descended from the same or nearly similar
parents, and are therefore in that degree closely related. Thus, community
in embryonic structure reveals community of descent. It will reveal this
community of descent, however much the structure of the adult may have been
modified and obscured; we have seen, for instance, that cirripedes can at
once be recognised by their larvae as belonging to the great class of
crustaceans. As the embryonic state of each species and group of species
partially shows us the structure of their less modified ancient
progenitors, we can clearly see why ancient and extinct forms of life
should resemble the embryos of their descendants,--our existing species.
Agassiz believes this to be a law of nature; but I am bound to confess that
I only hope to see the law hereafter proved true. It can be proved true in
those cases alone in which the ancient state, now supposed to be
represented in many embryos, has not been obliterated, either by the
successive variations in a long course of modification having supervened at
a very early age, or by the variations having been inherited at an earlier
period than that at which they first appeared. It should also be borne in
mind, that the supposed law of resemblance of ancient forms of life to the
embryonic stages of recent forms, may be true, but yet, owing to the
geological record not extending far enough back in time, may remain for a
long period, or for ever, incapable of demonstration.
Thus, as it seems to me, the leading facts in embryology, which are second
in importance to none in natural history, are explained on the principle of
slight modifications not appearing, in the many descendants from some one
ancient progenitor, at a very early period in the life of each, though
perhaps caused at the earliest, and being inherited at a corresponding not
early period. Embryology rises greatly in interest, when we thus look at
the embryo as a picture, more or less obscured, of the common parent-form
of each great class of animals.
Rudimentary, atrophied, or aborted organs. -- Organs or parts in this
strange condition, bearing the stamp of inutility, are extremely common
throughout nature. For instance, rudimentary mammae are very general in
the males of mammals: I presume that the 'bastard-wing' in birds may be
safely considered as a digit in a rudimentary state: in very many snakes
one lobe of the lungs is rudimentary; in other snakes there are rudiments
of the pelvis and hind limbs. Some of the cases of rudimentary organs are
extremely curious; for instance, the presence of teeth in foetal whales,
which when grown up have not a tooth in their heads; and the presence of
teeth, which never cut through the gums, in the upper jaws of our unborn
br /> calves. It has even been stated on good authority that rudiments of teeth
can be detected in the beaks of certain embryonic birds. Nothing can be
plainer than that wings are formed for flight, yet in how many insects do
we see wings so reduced in size as to be utterly incapable of flight, and
not rarely lying under wing-cases, firmly soldered together!
The meaning of rudimentary organs is often quite unmistakeable: for
instance there are beetles of the same genus (and even of the same species)
resembling each other most closely in all respects, one of which will have
full-sized wings, and another mere rudiments of membrane; and here it is
impossible to doubt, that the rudiments represent wings. Rudimentary
organs sometimes retain their potentiality, and are merely not developed:
this seems to be the case with the mammae of male mammals, for many
instances are on record of these organs having become well developed in
full-grown males, and having secreted milk. So again there are normally
four developed and two rudimentary teats in the udders of the genus Bos,
but in our domestic cows the two sometimes become developed and give milk.
In individual plants of the same species the petals sometimes occur as mere
rudiments, and sometimes in a well-developed state. In plants with
separated sexes, the male flowers often have a rudiment of a pistil; and
Kolreuter found that by crossing such male plants with an hermaphrodite
species, the rudiment of the pistil in the hybrid offspring was much
increased in size; and this shows that the rudiment and the perfect pistil
are essentially alike in nature.
An organ serving for two purposes, may become rudimentary or utterly
aborted for one, even the more important purpose; and remain perfectly
efficient for the other. Thus in plants, the office of the pistil is to
allow the pollen-tubes to reach the ovules protected in the ovarium at its
base. The pistil consists of a stigma supported on the style; but in some
Compositae, the male florets, which of course cannot be fecundated, have a
pistil, which is in a rudimentary state, for it is not crowned with a
stigma; but the style remains well developed, and is clothed with hairs as
in other compositae, for the purpose of brushing the pollen out of the
surrounding anthers. Again, an organ may become rudimentary for its proper
purpose, and be used for a distinct object: in certain fish the
swim-bladder seems to be rudimentary for its proper function of giving
buoyancy, but has become converted into a nascent breathing organ or lung.
Other similar instances could be given.
Rudimentary organs in the individuals of the same species are very liable
to vary in degree of development and in other respects. Moreover, in
closely allied species, the degree to which the same organ has been
rendered rudimentary occasionally differs much. This latter fact is well
exemplified in the state of the wings of the female moths in certain
groups. Rudimentary organs may be utterly aborted; and this implies, that
we find in an animal or plant no trace of an organ, which analogy would
lead us to expect to find, and which is occasionally found in monstrous
individuals of the species. Thus in the snapdragon (antirrhinum) we
generally do not find a rudiment of a fifth stamen; but this may sometimes
be seen. In tracing the homologies of the same part in different members
of a class, nothing is more common, or more necessary, than the use and
discovery of rudiments. This is well shown in the drawings given by Owen
of the bones of the leg of the horse, ox, and rhinoceros.
It is an important fact that rudimentary organs, such as teeth in the upper
jaws of whales and ruminants, can often be detected in the embryo, but
afterwards wholly disappear. It is also, I believe, a universal rule, that
a rudimentary part or organ is of greater size relatively to the adjoining