Another favourite argument is the fact of “Pædogenesis.” Certain animals, such as Amphioxus lanceolatus, Peripatus, and certain Medusæ, are very frequently brought forward as examples of persistent primitive stages and “transitional connecting links.” But considered from the point of view of Pædogenesis, they all assume quite a different aspect, and seem rather to represent very highly evolved species, and to be, not primitive forms, but conservative and regressive forms. Pædogenesis is the phenomenon exhibited by a number of species, which may stop short at one of the stages of their embryonic or larval development, become sexually mature, and produce [pg 124] offspring without having attained their own fully developed form.
Another argument is the old, suggestive, and really important one urged by Kölliker, that “inorganic nature shows a natural system among minerals (crystals) just as much as animals and plants do, yet in the former there can be no question of any genetic connection in the production of forms.”
Yet another argument is found in the occurrence of “inversions” and anomalies in the palæontological succession of forms, which to some extent upsets the Darwinian-Haeckelian genealogical trees. (Thus there are forms in the Cambrian whose alleged ancestors do not appear till the Silurian. Foraminifera and other Protozoa do not appear till the Silurian.)
From embryology in particular, as elsewhere in general, we read the “fundamental biogenetic law,” that evolution is from the general to the special, from the imperfect to the more perfect, from what is still indefinite and exuberant to the well-defined and precise, but never from the special to the special. According to Hamann's hypothesis we must think of evolution as going on, so to speak, not about the top but about the bottom. The phyla or groups of forms are great trunks bearing many branches and twigs, but not giving rise to one another. Still less do the little side branches of one trunk bear the whole great trunk of another animal or plant phylum. But they all grow from the same roots among the primitive forms of life. Unicellulars these must [pg 125] have been, but not like our “Protists.” They should be thought of as primitive forms having within themselves the potentialities of the most diverse and widely separate evolution-series to which they gave rise, as it were, along diverging fan-like rays.
It would be instructive to follow some naturalist into his own particular domain, for instance a palæontologist into the detailed facts of palæontology, or an embryologist into those of embryology, in order to learn whether these corroborate the assumptions of the Theory of Descent or not. It is just in relation to these detailed facts that criticisms or even denials of the theory have been most frequent. Koken, otherwise a convinced supporter of the theory, inquires in his “Vorwelt,” apropos of the tortoises, what has become of the genealogical trees that were scattered abroad in the world as proved facts in the early days of Darwinism. He asserts, in regard to Archæopteryx, the instance which is always put forward as the intermediate link in the evolution of birds, that it does not show in any of its characters a fundamental difference from any of the birds of to-day, and further, that, through convergent development under similar influences, similar organs and structural relations result, iterative arrangements which come about quite independently of descent. He maintains, too, that the principle of the struggle for existence is rather disproved than corroborated by the palæontological record.
In embryology, so competent an authority as [pg 126] O. Hertwig—himself a former pupil of Haeckel's—has reacted from the “fundamental biogenetic law.” His theory of the matter is very much that of Hamann which we have already discussed; development is not so much a recapitulation of finished ancestral types as the laying down of foundations after the pattern of generalised simple forms, not yet specialised; and from these foundations the special organs rise to different levels and grades of differentiation according to the type.[29] But we must not lose ourselves in details.
Looking back over the whole field once more, we feel that we are justified in maintaining with some confidence that the different pronouncements in regard to the detailed application and particular features of the Theory of Descent, and the different standpoints that are occupied even by evolutionists, are at least sufficient to make it obvious that, even if evolution and descent have actually taken place, they have not run so simple and smooth a course as the over-confident would have us believe; that the Theory of Descent rather emphasises than clears away the riddles and difficulties of the case, and that with the mere corroboration of the theory we shall have gained only something relatively external, a clue to creation, which does not so much solve its problems as restate them. The whole criticism of the “right wing,” from captious objections to actual denials, proves this indisputably. And it seems likely that in the course of time a sharpening of [pg 127] the critical insight and temper will give rise to further reactions from the academic theory as we have come to know it.[30] On the other hand, it may be assumed with even greater certainty that the general evolutionist point of view and the great arguments for descent in some form or other will ultimately be victorious if they are not so already, and that, sooner or later, we shall take the Theory of Descent in its most general form as a matter of course, just as we now do the Kant-Laplace theory.