VARIATION IN SPACE IN GEOLOGICAL TIMES 147 



and local or regional races due to the circumstances 

 of the environment, and which sometimes are 

 perceptibly removed from the original type. These 

 forms, too often considered by the naturalists de- 

 scribing them as distinct species, are nearly always 

 linked together by imperceptible transitions. But 

 it is important to remark that these groups of forms, 

 assembled round a central type, like the different 

 stars round a nebula, have a veritable objective 

 reality, and almost always remain sharply separated 

 from the neighbouring groups, if we neglect a few 

 extremely rare cases of hybridization. These are 

 the groups which answer, or should answer, to the 

 true definition of the species characterized at once 

 morphologically, genetically, and geographically. 

 It is in this sense that we are able to affirm that 

 the great species do not pass gradually from one to the 

 other, either in existing nature or at any of the 

 early epochs in the life of the globe. 



This conclusion, which could easily have been 

 foreseen a priori, is, moreover, in no way contrary 

 to the descent hypothesis. If this hypothesis is 

 correct, we must admit that the transformation 

 of species must have corresponded to phenomena 

 of the same order as those to which we owe our 

 local races that is to say, to a prolonged isolation 

 in very different conditions of environment. It is 

 already remarkable to see how these local races in 

 living nature at times depart from the original 

 type. 



It would be irrational to suppose that this 

 departure could, in one epoch, go as far as a com- 

 plete separation of two great species ; and observa- 



