80 THE FOLLOWERS OF GEOFFKOY 



In the first of them he points out how neither position 

 nor function has proved altogether sufficient to establish 

 homologies. In the early days anatomists were guided by 

 form ; when form failed them, they traced an organ in its 

 changes throughout the series of animals by considering its 

 function. This method was satisfactory enough as regards 

 the organs of the nutritive life. But in the organs of the life 

 of relation, in the nervous system, the functions of the parts 

 were difficult to discover, and their form very changeful. 

 Hence a new principle was required, and Serres found it in 

 the thought which he probably owed to the German trans- 

 cendentalists (see Chap. VII.), that the permanent structure of 

 the lower animals could be compared with phases in the 

 development of the higher, and particularly of man, or, as he 

 put it, that comparative anatomy was often only a fixed and 

 permanent anthropogeny, and anthropogeny a fugitive 

 and transitory comparative anatomy (xi., p. 106). 



" In rising towards the first formations," he writes, "trans- 

 cendental anatomy recognised that one and the same organ, 

 however complicated its definitive form might be, repeated 

 in its transitory states the organic simplicities of the lower 

 classes. Thus the primitive heart of birds was first of all 

 a canal, then a pocket or single cavity, then finally the 

 complex organ of the class. Comparative anatomy was 

 thus seen to be repeated and reproduced by embryogeny" 

 (xii, p. 85). 



His explanation of the fact of repetition is that, "in 

 animals belonging to the lower classes the formative force, 

 whatever it may be, has a less energetic impulsion than in 

 the higher animals, and hence the organs pass through only a 

 part of the transformations which those of the higher forms 

 undergo; and it is for this reason that they show permanently 

 the organic dispositions which are only transitory in the 

 embryo of man and the higher Vertebrates. Hence these 

 double aortas, these double vena:: cav;e which one observes 

 more or less constantly among reptiles " (xxi., p. 48). 



The number of stages in embryogeny is proportionate to 

 the complexity of the adult ; the younger the embryo the 

 simpler its organs such is the general formula of the relation 

 between the embryo and the adult. But here in Serres' 



