182 THE LESSON OF EVOLUTION 



individual choice continue in the same direction, and 

 without this the new variation would not progress. 

 Definite variation seems to be the only solution of 

 the difficulty. New characters certainly arise in 

 some way, and they are preserved by isolation. 



There are other interesting facts about these 

 cormorants which ought not to be passed over. All 

 of them are well adapted for catching fish with their 

 long hooked bill, and they swim and dive by means 

 of their broadly webbed feet, never opening their 

 wings under water. But when we come to examine 

 the differences which divide the sub-genera, it is diffi- 

 cult to suggest any utilitarian origin for them. For 

 example , most of the cormorants have twelve feathers 

 in their tail, and it might be thought that so constant 

 a character must, in some way, be useful and due 

 to natural selection. But a few have constantly 

 fourteen feathers in the tail, and so the idea of 

 twelve being of special importance breaks down. 

 The bills also are much stouter in some than in 

 others, and this character cannot be correlated with 

 any difference in food. 



I need not press the subject any further. It 

 must, I think, be allowed that a large number of 

 characters are not useful, and cannot have been pre- 

 served by natural selection. 



Physiological Isolation. This is when certain 

 individuals of a species are prevented from inter- 

 crossing with other individuals by physiological 

 causes, although they freely mix together during the 

 breeding season. The simplest form of it is found 

 in the lowest plants and animals which have no sex, 





