232 POPULATIONS OF THE SEA 



each descended from a single ancestor will compete with each other 

 and the most efficient will survi^•e. There will be no opportunity 

 for the spread of mutations from one lineage to another. Such 

 species must depend entirely for their e\^olution on the occurrence 

 of mutation in the separate lineages. The scheme I have outlined, 

 besides according with the known ecology of animals, has the 

 advantage of accounting for both divergent and successional 

 evolution. It also allows us to understand how the many apparently 

 nonadaptive characters that are found in natural species, and 

 often distinguish one species from another, can be evolved in the 

 presence of selection. 



There is another character of many terrestrial en\'ironments 

 that seems important for a comparison of the process of e\'olution 

 in them and in the deep sea. We are living in an exceptional period 

 of the world's history. It is only 12,000-20,000 years since the 

 retreat of the ice at the end of the last glacial period, and on both 

 the geological and evolutionary time scales that is a very short 

 period. Whether the Ice Age is wholly over, or the present is an 

 interglacial period after which the ice will again advance, is for 

 our present discussion unimportant. In either case it is clear that 

 climatic changes have been great and rapid in the last 20,000 

 years, much more so than in the longer, more stable parts of the 

 world's history. In temperate regions the most important climatic 

 changes have been in temperature; in the tropics temperature has 

 probably not varied greatly either during the Ice Age or after it, 

 but there have been large variations in rainfall. The amount of 

 rain is in the tropics the most important condition controlling the 

 distribution of animals, and its variations must have had almost 

 as marked effects on evolution as those in temperature in the 

 cooler regions. 



Animal evolution has undoubtedly been largely affected by this 

 unusual instability of climate. The most obvious effect has been 

 to increase the rate of evolution. Change of the conditions to 

 which an animal is exposed will always lead to rapid change in 

 its adaptations, provided that it is able to survive the change, for 

 the direction of selection will be altered and the animal's charac- 

 ters will alter in response to the new selection. In fact, stability or 



