GENESIS OF MAN. 25 



that the different branches of the main stem have in our time 

 spread so widely, and become so far differentiated by adaptive in- 

 fluences, that even their embryos have lost many of the original 

 traces of relationship. The important fact remains that within the 

 vertebrate type, as within other types, the embryonic stages cor- 

 respond with wonderful accuracy to the successively ascending 

 classes and orders established for that type. 



The law of Von Baer, expressed in the most general terms, as 

 laid down by himself, is in these words : " The development of an 

 individual of a definite animal form is determined by two relations: 

 first, by a progressive development of the animal body through in- 

 creasing histological and morphological differentiation (Sonderung)', 

 secondly, through simultaneous progressive development from a 

 more general form of the type to a more special. The degree of 

 development of the animal body consists in a greater or less 

 amount of heterogeneity of the elementary parts and of the indi- 

 vidual components of a composite apparatus ; in a word, in the 

 greater histological and morphological differentiation. The type, 

 on the contrary, is the fundamental relation of the organic elements 

 and organs." 



Upon this important law, Haeckel puts the new interpretation 

 that the " type " of Von Baer is the representative of the law of 

 heredity of Darwin, (the vis centripeta of Goethe), while the " degree 

 of development " means neither more nor less than his law of 

 adaptation (Goethe's vis centrifnga). 



The parallelism which is found to exist between the facts of 

 ontogenesis and the facts of phylogenesis, between the embryonic 

 forms of higher, and the adult forms of lower, organisms, is one of 

 the most astonishing discoveries which science has ever made. 

 It is one which would have been least likely ever to be reached 

 by conjecture or by any form of a priori reasoning. There was 

 but one possible mode of reaching this truth, and this was by 

 long and patient investigation of the minutest objects and most 

 occult phenomena, without the aid even of a " working hypoth- 

 esis." 



Such a truth must have a meaning. This meaning Von Baer 

 himself never realized and, when pointed out to him by others, never 

 accepted. Yet I venture to predict that no unbiased reader of 

 Haeckel's Anthropogenie will any longer doubt the justice of his 



