532 EVOLUTION OF BIRDS xvm. n 



1 1 . The development of variety of bird life 



This and similar evidence from the study of the relatively recent 

 evolution of birds might be summarized by saying that the production 

 of variety of animal types is largely due to the interplay of biotic 

 factors. The particular characteristics that each population acquires 

 depend partly on physical and geographical conditions, but the 

 stimulus to change, or the check on change, comes from the inter- 

 action of the animals and plants with each other. 



It has already been suggested that the tendencies to increase and to 

 vary are important among these factors influencing animal evolution 

 and there is evidence that both have been at work in the development 

 of the population of Galapagos finches and other animals. It is safe to 

 say that there are more and varied finches, tortoises, iguanas, and 

 mocking-birds than there were when each arrived. The other factors 

 that we are now able to isolate, by their absence in this case, are the 

 competitors and the predators. In more fully developed continental 

 populations these probably tend to limit the development of variety, 

 allowing only those individuals of a population showing the mean or 

 'normal' structure and behaviour to survive. Extreme variants that 

 venture to brave the enemies and seek new habitats are eliminated. 

 Variation will arise when these checks are weak. In a crowded habitat 

 this may occur as a result of some peculiar swing of the elaborately 

 balanced interacting system of biotic factors, or by some external 

 physical change. We still do not know which of these is the more 

 important in producing the changes of these complicated mainland 

 populations, but the evolutionary laboratories provided by the vol- 

 canoes of the Galapagos and other islands suggest that evolutionary 

 change does not follow only on climatic or other physical changes. 

 A single population will become divided into several distinct ones 

 by its own tendencies to growth and variation, given absence of 

 competitors and predators and some means of isolating the animals 

 in different parts of the range. 



