xxxii. i2 INFLUENCE OF CLIMATIC CHANGE 777 



and migrated into the area to meet the new conditions left after the 

 'revolution'. 



In the past it has been usual to try to find rather simple correlations 

 of this sort. We are told that emergence of land vertebrates was due 

 to drying up of large areas of sea during the Devonian, that the 

 mammals emerged because of the colder conditions at the end of the 

 Cretaceous, or the horses because of the appearance of wide plains in 

 the Miocene. For the reasons already given we must regard such 

 suggestions with suspicion, especially when they relate to conditions 

 extending over a long period of time. There are, however, certainly 

 some valid correlations of climatic and faunistic changes, especially in 

 relatively recent periods. Thus the finding in Britain of woolly mam- 

 moths, cave bears, and other animals to be expected in a cold climate 

 may reasonably be associated with advances of the ice cap (p. 572). 

 Indeed we have evidence of the climatic change independent of the 

 animal remains. No doubt change of climate has been one of the 

 variable factors that has led to the continual change of vertebrate life, 

 which we are seeking to understand. It is probable, however, that 

 animal populations change their character independently of any climatic 

 change. It is not easy to find critical situations to test this belief, but 

 examples such as the faunas of the Galapagos and other islands (p. 524) 

 suggest that diversity can arise as animals explore the possibilities of 

 their environment, especially if there are factors that divide up a 

 population into a number of nearly isolated units. 



Often a population undergoes an 'adaptive radiation', branching out 

 to form a number of types, each suited for a particular environment or 

 niche. The phenomenon is so widespread that it suggests a type of 

 evolution common at least to many populations. The conception of 

 adaptive radiation originally put forward by Osborn was that each 

 'stem form' (of mammals) diverged in five directions, giving cursorial, 

 fossorial, scansorial, volant, and aquatic types. These are, of course, 

 only particular aspects of the radiation. When we examine, say, the 

 Galapagos finches, or the marsupials, we obtain the strong impression 

 that members of a particular animal population seek out a variety of 

 new habitats, and gradually become suited for them, until a range of 

 new types is thus produced. This is the history of each of the groups of 

 vertebrates, they radiate into many different types and then disappear. 



12. Convergent and parallel evolution 



A remarkable fact that has appeared many times in our survey is that 

 during these radiations similar features repeatedly appear in distinct 



