THE ORIGIN OF SPECIES AND GENERA 279 



uous and, so to speak, normal, in which phyletic 

 branches once formed develop slowly, by gradual 

 mutations following certain laws which fatally 

 lead them to senility and extinction ; the other 

 intermittent, in which new branches are evolved 

 by divergence from branches which are older and 

 have already more or less experienced evolution. 

 This divergence seems, moreover, to be affected by 

 at least two processes : one of them geographical 

 isolation of certain and of relatively slow action, but 

 able to lead to considerable divergences, which 

 assume, according to the lapse of time, the value 

 of local races, species, and genera ; the other less 

 clearly perceptible, but with a greater rapidity of 

 action, of which the explosions or sudden creations 

 of species studied by de Vries in existing plants 

 may doubtless give us an idea. 



We are able to conceive by the aid of both these 

 processes the differentiation of species, of genera, 

 and, perhaps, even of families, by recalling to mind 

 the almost unlimited duration of geological times. 

 But we have to confess that at the present day we 

 are utterly unable to see and even to explain other- 

 wise than by simple theoretical views the funda- 

 mental divergences which separate the orders, 

 classes, and great ramifications of the animal 

 kingdom. 



