Hence, remembering the perpetual intrusions of organisms on one another's modes of life, often widely different; and remembering that these intrusions have been going on from the beginning; we shall be prepared to find that the general law of embryonic parallelism is qualified by irregularities which are mostly small, in many cases considerable, and occasionally great. The hypothesis of evolution accounts for these: it does more—it implies the necessity of them.
§ 130. The substitutions of organs and the suppressions of organs, are among those secondary embryological phenomena which harmonize with the belief in evolution but cannot be reconciled with any other belief. Some embryos, during early stages of development, possess organs that afterwards dwindle away, as there arise other organs to discharge the same functions. And in other embryos organs make their appearance, grow to certain points, have no functions to discharge, and disappear by absorption.
We have a remarkable instance of substitution in the temporary appliances for respiration, which some embryos exhibit. During the first phase of its development, the mammalian embryo possesses a system of blood-vessels distributed over what is called the area vasculosa—a system of vessels homologous with one which, among fishes, serves for aërating the blood until the permanent respiratory organs come into play. Now since this system of blood-vessels, not being in proximity to an oxygenated medium, cannot be serviceable to the mammalian embryo during development of the lungs, as it is serviceable in the embryo-fish during development of the gills, this needless formation of it is unaccountable as a result of design. But it is quite congruous with the supposition that the mammalian type arose out of lower vertebrate types. For in such case the mammalian embryo, passing through states representing in a general way those which its remote ancestors had in common with the lower Vertebrata, develops this system of vessels in like manner with them. An instance more significant still is furnished by certain Amphibia. One of the facts early made familiar to the natural-history student is that the tadpole breathes by external branchiæ, and that these, needful during its aquatic life, dwindle away as fast as it develops the lungs fitting it for terrestrial life. But in one of the higher Amphibia, the viviparous Salamander, these transformations ordinarily undergone during the free life of the larva, are undergone by the embryo in the egg. The branchiæ are developed though there is no use for them: lungs being substituted as breathing appliances before the creature is born.
Even more striking than the substitutions of organs are the suppressions of organs. Mr. Darwin names some cases as "extremely curious; for instance, the presence of teeth in fœtal whales, which when grown up have not a tooth in their heads;... It has even been stated on good authority that rudiments of teeth can be detected in the beaks of certain embryonic birds." Irreconcilable with any teleological theory, these facts do not even harmonize with the theory of fixed types which are maintained by the development of all the typical parts, even where not wanted; seeing that the disappearance of these incipient organs during fœtal life spoils the typical resemblance. But while to other hypotheses these facts are stumbling-blocks, they yield strong support to the hypothesis of evolution.
Allied to these cases, are the cases of what has been called retrograde development. Many parasitic creatures and creatures which, after leading active lives for a time, become fixed, lose, in their adult states, the limbs and senses they had when young. It may be alleged, however, that these creatures could not secure the habitats needful for them, without possessing, during their larval stages, eyes and swimming appendages which eventually become useless; that though, by losing these, their organization retrogresses in one direction, it progresses in another direction; and that, therefore, they do not exhibit the needless development of a higher type on the way to a lower type. Nevertheless there are instances of a descent in organization, following an apparently-superfluous ascent. Mr. Darwin says that in some genera of cirripedes, "the larvæ become developed either into hermaphrodites having the ordinary structure, or into what I have called complemental males, and in the latter, the development has assuredly been retrograde; for the male is a mere sack, which lives for a short time, and is destitute of mouth, stomach, or other organ of importance, excepting for reproduction."
§ 130a. But now let us contemplate more closely the energies at work in the unfolding embryo, or rather the energies which the facts appear to imply.
Whatever natures we ascribe to the hypothetical units proper to each kind of organism, we must conclude that from the beginning of embryonic development, they have a proclivity towards the structure of that organism. Because of their phylogenetic origin, they must tend towards the form of the primitive type; but the superposed modifications, conflicting with their initial tendency, must cause a swerving towards each successively higher type. To take an illustration:—If in the germ-plasm out of which will come a vertebrate animal there is a proclivity towards the primitive piscine form, there must, if the germ-plasm is derived from a mammal, be also from the outset a proclivity towards the mammalian form. While the initial type tends continually to establish itself the terminal type tends also to establish itself. The intermediate structures must be influenced by their conflict, as well as by the conflict of each with the proclivities towards the amphibian and reptilian types. This complication of tendencies is increased by the intervention of several other factors.
There is the factor of economy. An embryo in which the transformations have absorbed the smallest amount of energy and wasted the smallest amount of matter, will have an advantage over embryos the transformations of which have cost more in energy and matter: the young animal will set out with a greater surplus of vitality, and will be more likely than others to live and propagate. Again, in the embryos of its descendants, inheriting the tendency to economical transformation, those which evolve at the least cost will thrive more than the rest and be more likely to have posterity. Thus will result a continual shortening of the processes. We can see alike that this must take place and that it does take place. If the whole series of phylogenetic changes had to be repeated—if the embryo mammal had to become a complete fish, and then a complete amphibian, and then a complete reptile, there would be an immense amount of superfluous building up and pulling down, entailing great waste of time and of materials. Evidently these abridgments which economy entails, necessitate that unfolding embryos bear but rude resemblances to lower types ancestrally passed through—vaguely represent their dominant traits only.
From this principle of economy arise several derivative principles, which may be best dealt with separately.
§ 130b. In some cases the substitution of an abridged for an unabridged course of evolution causes the entire disappearance of certain intermediate forms. Structural arrangements once passed through during the unfolding are dropped out of the series.