The science of life : fully illustrated in tone and line and including many diagrams

BOOK 3

land animals were created as land animals. But it at once becomes pregnant with meaning if we accept the fact of Evolution, for then we can understand that snakes and hens and human beings and all other airbreathing vertebrates are fundamentally fishlike, that they start on the fishward road and turn away from it towards their higher structural achievement. When they reproduce the old disposition asserts itself; they start towards the old water-way and turn aside towards the uplands. Because of that recurrent urge each individual animal repeats within its individual cycle of life these uneffaced tendencies from the remote part of its race. In Amphibia the recapitulation is much more thoroughgoing ; they have not only the clefts but the gills, and most of them actually do breathe by means of gills in their early tadpole stages, and physiologically are indeed fish.

Nearly half a century later, Haeckel, looking at the facts of embryology in the light of evolutionary ideas, broadened and reformulated and perhaps rather exaggerated von Baer’s law. Haeckel’s revision was this, that every animal in the course of its individual development tends to recapitulate the development of the race; and from this time onwards the facts.on which the law is based have been called the facts of recapitulation. But it is a general and not a complete recapitulation. Evolution can affect every part of a life-cycle, and if a stage wastes much time or energy, Nature, who is no historian, will abbreviate it or cut it out quite ruthlessly.

Exactly how far Haeckel’s law takes uswhat are its limitations ; whether recapitulation ever shows us an animal’s adult ancestors; whether present development ever does more than recapitulate ancestral development ; what is the cause of recapitulation; and why some of the characters and structures are regularly recapitulated in development, others only occasionally or not at all—all this we cannot here discuss. What we are here concerned with are the positive, visible facts. Tens of thousands of animals do recapitulate the past during their development—do, without any apparent advantage in so doing, show organs and constructions which. occur elsewhere in the adults of less specialized creatures : and in none of these tens of thousands of cases is this departure intelligible save on the view that in so doing they are repeating phases that were once final forms in the earlier evolution of the race.

There is probably no single case of

224

THE SCIENCE OF LIFE

“CHAPTER 3

development among many-celled animals which does not show some recapitulatory feature. Even the origin of all sexually produced individuals in a single cell, the fertilized egg, in a certain sense recapitulates the origin of many-celled from singlecelled forms. But there are plenty of more definite examples. Every human being and every other vertebrate at a certain stage of his or her development has an unjointed notochord in place of the future jointed backbone, even though in all save the lampreys and certain fish this notochord-rod vanishes entirely. And the most primitive chordate, Amphioxus, has a notochord all its life long and never develops a backbone.

We have already spoken of the extraordinary transformation that overtakes the young sea-squirt. Here the sedentary adult animal, bearing only faintest indications of its real relationship, passes its early life in a tadpole-like form which shows all the salient points of the chordate plan—the notochord, the tubular nervous system along the back, and the gill-slits. Ifthe gelatinous sea-squirt is not a degenerate chordate, why should its larval form be so entirely chordate ? What creative idea is served by this hesitation in development ?

A much less all-pervading example, but none the less a very pretty one, we have already cited in the early horse-stock. The three-toed Merychippus had an elaborate grinding pattern on its permanent premolar teeth ;. but the pattern of the milk-teeth was simpler, like the pattern of the permanent teeth in geologically earlier and more primitive horse-ancestors. Here we can actually put side by side the adult ancestral form and its young descendant recapitulating it before passing on to the more highly evolved structure.

A beautiful example of recapitulation comes in the life-history of the common feather-star. This is an echinoderm belonging to the crinoid class. ‘The great majority of the class spend their adult lives rooted to the bottom; they are the stalked sea-lilies which wave their graceful, branching arms far down in the abyss, and in the past they were so abundant as to have built up whole layers of rock with their skeletons. From such forms as these the freely swimming feather-stars have evolved. But the featherstar, too, begins its adult existence with a fixed stage. For some weeks it lives and grows rooted to the bottom like a sea-lily. Only later does it abandon this sea-lily guise, break off from its stalk (which is left