32 THE EVOLUTION OF MAN. 



corresponds in its essential characteristics with that of all 

 other higher Vertebrates ; and, therefore, also of Man. 

 The three higher classes of Vertebrates, Mammals, Birds, 

 and Reptiles (Lizards, Snakes, Turtles, etc.), are from the 

 beginning of their individual development so surprisingly 

 similar in all essential features of their bodily structure, 

 especially in the earlier stages, that for a long while it is 

 impossible to distinguish them. (Of. Plates VI. and VII.) 

 It has long been known that the accurate study of the 

 evolution of the embryo of the Bird, which is most readily 

 obtained as the subject of research, is all that is necessary 

 in order to learn the essentially similar mode of evolution 

 of Mammals, therefore also of Man. Even as early as the 

 middle and the end of the seventeenth century, when 

 human embryos, as well as those of all other Mammals, 

 began to be examined in their earlier stages, this most 

 important fact was soon recognized. It is of the greatest 

 importance, both for theoretical and for practical purposes. 

 Conclusions of the highest importance to the theory of 

 evolution may be drawn from the similarity of structure 

 of the embryos of widely differing animals. This simi- 

 larity is invaluable in practical ontogenetic research, 

 because the ontogeny of Birds, which is accurately known, 

 most completely supplements and explains the embryology 

 of Mammals, which has been but imperfectly studied. 

 Hen's eggs can be obtained at all times and in any quan- 

 tity, and by hatching them artificially the evolution of 

 the embryo may be traced step by step. On the other 

 hand, it is much more difficult to study the evolution of 

 Mammals, because the embryo of these does not develop 

 in a large egg that has been laid, or, in other words, in an 



