"The conclusion here reached is that, whereas larval development must retain traces (it may be very faint) of ancestral stages of structure because they are built out of ancestral stages, embryonic development need not necessarily do so, and very often does not; that embryonic development in so far as it is a record at all, is a record of structural features of previous larval stages. Characters which disappear during free life disappear also in the embryo, but characters which though lost by the adult are retained in the larva may ultimately be absorbed into the embryonic phase and leave their traces in embryonic development."[[48]]
To set forth the evidence justifying this view would encumber too much the general argument. Towards elucidation of such irregularities let me name two factors which should I think be taken into account.
Abridgment of embryonic stages cannot go on uniformly with all disused organs. Where an organ is of such size that progressive diminution of it will appreciably profit the young animal, by leaving it a larger surplus of unused material, we may expect progressive diminution to occur. Contrariwise, if the organ is relatively so small that each decrease will not, by sensibly increasing the reserve of nutriment, give the young animal an advantage over others, decrease must not be looked for: there may be a survival of it even though of very ancient origin.
Again, the reduction of a superfluous part can take place only on condition that the economy resulting from each descending variation of it, is of greater importance than are the effects of variations simultaneously occurring in other parts. If by increase or decrease of any other parts of the embryo, survival of the animal is furthered in a greater degree than by decrease of this superfluous part, then such decrease is unlikely; since it is illegitimate to count upon the repeated concurrence of favourable variations in two or more parts which are independent. So that if changes of an advantageous kind are going on elsewhere in the embryo a useless part may remain long undiminished.
Yet another cause operates, and perhaps cooperates. Embryonic survival of an organ which has become functionless, may readily happen if, during subsequent stages of development, parts of it are utilized as parts of other organs. In the words of Mr. J. T. Cunningham:—
"It seems to be a general fact that a structure which in metamorphosis disappears completely may easily be omitted altogether in embryonic development, while one which is modified into something else continues to pass more or less through its original larval condition." (Science Progress, July, 1897, p. 488.)
One more factor of considerable importance should be taken into account. A disused organ which entails evil because construction of it involves needless cost, may entail further evil by being in the way. This, it seems to me, is the reason why the fore-limbs of snakes have disappeared from their embryos. When the long-bodied lizard out of which the ophidian type evolved, crept through stiff herbage, and moved its head from side to side to find openings, there resulted alternate bends of its body, which were the beginnings of lateral undulations; and we may easily see that in proportion as it thus progressed by insinuating itself through interstices, the fore-limbs, less and less used for walking, would be more and more in the way; and the lengthening of the body, increasing the undulatory motion and decreasing the use of the fore-limbs, would eventually make them absolute impediments. Hence besides the benefit in economy of construction gained by embryos in which the fore-limbs were in early stages a little less developed than usual, they would gain an advantage by having, when mature, smaller fore-limbs than usual, leading to greater facility of locomotion. There would be a double set of influences causing, through selection, a comparatively rapid decrease of these appendages. And we may I think see also, on contemplating the kind of movement, that the fore-limbs would be more in the way than the hind limbs, which would consequently dwindle with such smaller rapidity as to make continuance of the rudiments of them comprehensible.
§ 131-132. So that while the embryonic law enunciated by von Baer is in harmony with the hypothesis of evolution, and is, indeed, a law which this hypothesis implies, the nonconformities to the law are also interpretable by this hypothesis.
Parallelism between the courses of development in species allied by remote ancestry, is liable to be variously modified in correspondence with the later ancestral forms passed through after divergence of such species. The substitution of a direct for an indirect process of formation, which we have reason to believe will show itself, must obscure the embryonic history. And the principle of economy which leads to this substitution produces effects that are very irregular and uncertain in consequence of the endlessly varied conditions. Thus several causes conspire to produce deviations from the general law.
Let it be remarked, finally, that the ability to trace out embryologic kinships and the inability to do this, occur just where, according to the hypothesis of Evolution, they should occur. We saw in [§ 100a] that zoologists are agreed in grouping animals into some 17 phyla—Mollusca, Arthropoda, Echinodermata, &c.—each of which includes a number of classes severally sub-divided into orders, genera, species. All the members of each phylum are so related embryologically, that the existence of a common ancestor of them in the remote past is considered certain. But when it comes to the relations among the archaic ancestors, opinion is unsettled. Whether, for instance, the primitive Chordata, out of which the Vertebrata emerged, have molluscan affinities or annelidan affinities, is still a matter in dispute. With regard to the origins of various other types no settled conclusions are held. Now it is clear that on tracing down each branch of the great genealogical tree, kinships would be much more manifest among the recently-differentiated forms than among those forms which diverged from one another in the earliest stages of organic life, and had separated widely before any of the types we now know had come into existence.