x Translator s Preface. 



field by the author of the present work in 1875. 

 It is possible, and indeed probable, that future 

 researches will show that other characters among 

 existing species can be traced to the same causes. 

 The great generalizations of embryology, which 

 science owes so largely to the researches of Karl 

 Ernst von Baer, bear to the theory of descent 

 the same relations that Kepler's laws bear to the 

 theory of gravitation. These last-named laws are 

 nothing more than generalized statements of the 

 motions of the planets, which were devoid of 

 meaning till the enunciation of the theory of 

 gravitation. Similarly the generalized facts of 

 embryology are meaningless except in the light 

 of the theory of descent. It has now become a 

 recognized principle in biology that animals in 

 the course of their development from the ovum 

 recapitulate more or less completely the phases 

 through which their ancestors have passed. The 

 practical application of this principle to the 

 determination of the line of descent of any 

 species or group of species is surrounded by 

 difficulties, but attempts have been made of late 

 years as by Haeckel in his Gastrula theory to 

 push the law to its legitimate consequences. In 

 this country Sir John Lubbock, in 1874, appealed 

 to the embryonic characters of larvae in support 

 of his views on the origin of insects. To the 

 author of this work (1876) is due the first appli- 

 cation of the principle of Ontogeny as revealing 



