RESUME 327 



The two processes, while simultaneous, differ en- 

 tirely in their natures and are never superimposed. 

 When we speak of higher and lower animals we do not 

 mean to convey the idea that the former are better 

 adapted than the latter to their conditions; it is in- 

 dubitable, on the contrary, that protozoa thrive as well 

 in their environment as vertebrates do in their own, 

 and that the most insignificant parasite enjoys in its 

 own sphere all the advantages possessed by higher 

 animals which are obliged to exert all their faculties 

 in order to preserve an existence beset by many dan- 

 gers. 



Whenever a species supersedes another species or 

 appears simultaneously with another, it would be 

 wrong to conclude that it is better adapted to its 

 mode of life than the species from which it sprang. 

 The criteria adopted in classification prove that, in 

 establishing the distinctions between species or 

 genera, no account is taken of the adaptive character 

 of structures. Moreover, the element of structure 

 itself is almost entirely disregarded when it comes to 

 estimating the degree of relationship between species ; 

 a structural likeness may result, in many cases, 

 from a convergence, from a similarity of functions, 

 whereas the all-important fact is a common 

 origin. In order to determine the phylogenetic 

 origin of an organ, we do not seek in lower an- 

 imals of the same type an organ which serves the 



