270 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



is confronted by a hiatus, explainable by a migration 

 of distant origin of the group under discussion. In 

 order to follow up the evolution of the branch, it 

 would, perhaps, be necessary to transport ourselves 

 into distant geographical centres, or often to un- 

 explored ones. I shall return later on in detail to 

 these phenomena of migrations, which have played 

 a most important part in the changes of the faunas 

 of geological times. It is probable that when the 

 exploration of the globe is more advanced, it will 

 be possible for palaeontologists to join end to end 

 these segments broken by the phenomena of migra- 

 tion, and to re-establish the continuity of the suc- 

 cessive mutations of the innumerable parallel 

 branches which represent the collective animal 

 world. Slow transformation will doubtless then 

 present itself as the most normal and the most 

 generalized process of palseontological evolution. 



But, if the mechanism we have just studied 

 offers an explanation of the regular development 

 of the species and genera of the same natural 

 branch, it does not seem, on the other hand, suf- 

 ficient to provoke the divergences necessary to 

 bring about the bifurcation of the various branches 

 of the same family, and those still more important 

 differences which must lead to the differentiation 

 of the orders, the classes, and the higher divisions 

 of the animal kingdom. It is here probably that 

 those more rapid mechanisms come into play, 

 which we will now endeavour to analyse. 



A first and still rather slow process of divergence 

 is offered to us by geographical isolation, combined 

 with changes of the environment. The study of 



