24 GENESIS OF MAN. 



II. 



ONTOGENESIS. 



THE primary law of ontogenesis and that which connects it 

 with the modern theory of development, is founded on the 

 discovery of Von Baer, that the different successive stages of the 

 embryonic development of the higher animals bear a singularly 

 close resemblance to certain lower animals in their adult state, and 

 that the embryos of many animals, and of man himself, in their 

 earlier stages, are scarcely distinguishable from one another. This 

 fact, as already remarked, was carefully studied by Von Baer, and 

 the successive stages of embryonic life systematically compared 

 and co-ordinated. In his great work on the History of Develop- 

 ment of Animals, (i 828-1 837), that distinguished embryologist has 

 given to the world the results of his exhaustive investigations. In 

 this work he announces that the theory of types founded by Cuvier 

 in 1 8 16, upon the facts of comparative anatomy, is confirmed by 

 those of embryology, and shows that the process of development, 

 which is the same for all the animals of any of the four types, is 

 different from those of different types. 



Haeckel does not gainsay the general truth of this statement, 

 but simply shows that it cannot be used as an argument against 

 the theory of descent, as Von Baer's investigations were confined to 

 fully differentiated animals of each type, and not extended to the 

 then little known Amphioxus and Ascidians, which later researches 

 have shown to constitute transition forms uniting two types. Be- 

 sides, as we shall see, however different the course of development 

 of different animals may be, the embryos of animals of higher 

 types pass through phases identical with the adult forms of some 

 of the lower types, though not of ethers, showing that the four 

 types of Cuvier and Von Baer — Radiates, Articulates, Mollusks 

 and Vertebrates — can neither be regarded as co-ordinate, nor as 

 regularly subordinated to each other. And this is not all : Von 

 Baer's own facts, and those of many embryologists, show that there 

 must be another type added to these four ; viz, the Protozoa, and that 

 from this the course of both phylogenetic and ontogenetic develop- 

 ment has been through the Worms directly to the Vertebrates, leav- 

 ing the remaining types untouched. It seems, therefore, simply 



