EMBRYOLOGY AND EVOLUTION
to learn the course of past evolution, but owing to the highly speculative character of such studies and to the differences of opinion as to what were original (palingenetic) and what were acquired (cænogenetic) characters, there gradually arose a widespread skepticism concerning the value of embryology for this purpose. Gegenbaur, in 1889, voiced the growing opinion among zoölogists in these words: “If we are compelled to admit that cænogenetic characters are intermingled with palingenetic, then we cannot regard ontogeny as a pure source of evidence regarding phyletic relationships. Ontogeny accordingly becomes a field in which an active imagination may have full scope for its dangerous play, but in which positive results are by no means everywhere to be attained. To attain such results the palingenetic and the cænogenetic phenomena must be sifted apart, an operation that requires more than one critical granum salis.” Since the time this was written there have been many other less moderate utterances to the same effect, some even declaring that there is no evidence that ontogeny ever recapitulates phylogeny and that Haeckel's “biogenetic law” has no foundation in fact.
But after all, these criticisms of certain details of the recapitulation theory have not destroyed the general and fundamental truth of that theory — namely, that many features of individual development repeat ancestral features. There are many remarkable and undoubted instances in which ontogeny repeats phylogeny and in which the relationships of organisms can be determined only by their embryological history. The most severe critics of Haeckel's “biogenetic law” do not deny this; their criticisms apply to details rather than to foundation principles.
It is certainly no mere accident that practically all animals being their individual existence as fertilized eggs; that before
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