268 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



It does not seem impossible to me to assign, 

 perhaps, a part to both these processes ; but we 

 must first establish a fundamental distinction 

 which seems to me to have been hitherto too 

 much neglected between the direct and, so to 

 speak, normal evolution of a branch already formed, 

 and the lateral variation which alone can lead to the 

 birth of new branches and to the divergence of 

 groups. 



By referring to the notions acquired above on 

 phyletic branches, we know, through the important 

 researches of Waagen, Neumayr, Branco, Mojsis- 

 ovics, Hyatt, etc., that it is possible to constitute, 

 by the aid of fossil animals, many series of forms 

 whose different terms or mutations, taken step by 

 step, stage by stage, and even zone by zone, are linked 

 to one another by almost imperceptible transitions. 

 The number of these series, established first in a 

 few families of the Molluscs, has to-day become very 

 considerable, and they have been found with identi- 

 cal characteristics in all the groups of the Inverte- 

 brates and of the Vertebrates. If we confine our- 

 selves to the comparison of immediate mutations, 

 the differences which separate them are very slight, 

 and appear too insignificant to deserve to be dis- 

 tinguished as species. But if we pass over a certain 

 number of these intermediate forms, and especially 

 if we happen to compare the extreme types of the 

 same branch, we notice differences important 

 enough to justify the separation, not only of species, 

 but sometimes even of perfectly legitimate genera. 

 Every paleontologist who has carefully studied 

 any group whatever of fossil animals has found 



