THE ORIGIN OF SPECIES AND GENERA 269 



himself face to face with these series of forms or 

 phyletic branches which split up by transverse seg- 

 ments into species and genera. I have above 

 shown remarkable examples of this among the 

 Proboscidians and the Anthracotheridse. 



It is true and this remark is important that 

 the species and the genera thus formed by the 

 direct and normal evolution of a branch always 

 remain very closely related to each other, and do 

 not present differences considerable enough for 

 them to be ranked as distinct natural families. 

 It must also be observed that this evolution seems 

 to take place in, so to speak, a spontaneous manner, 

 independent of the action of modifying causes 

 derived from the external environment. The pro- 

 cess of slow variation thus forces itself upon every 

 observer as the general rule of the direct evolution of 

 phyletic branches. 



Let us now follow up the lot of these natural 

 series, on the one hand, to their end, and, on the 

 other, towards their beginning. I have already 

 said, with regard to the causes of the extinction of 

 species, that the duration of these branches is more 

 or less long, but that always, after having obeyed 

 the laws of increase in size and of progressive 

 specialization, they abruptly ended in extinction 

 without issue. We must, of course, make excep- 

 tion of those branches which are evolving before 

 our eyes, and which comprise all the animals of 

 the existing fauna. At their lower part, phyletic 

 branches may also be followed for a longer or shorter 

 time, but they nearly always end in an abrupt way ; 

 or, rather, they appear to do so, for the observer 



