REPTILES AND AMPHIBIANS 131 



Very few of them adapted themselves to the new 

 conditions. 



Physical conditions are bound to modify a group 

 of animals when they change themselves, as we saw 

 in the first chapter. This applies not only to changes 

 of climate, but to volcanic changes, the drying-up of 

 seas, the sinking of continents, and the folding of the 

 earth's crust to form mountains. It applies also to 

 modifications caused by human culture. Most of these 

 factors may come into operation gradually, and modify 

 rather than destroy species. In fact, even civilisation, 

 which acts comparatively quickly, can modify species 

 of animals if the nature of them is such that only slight 

 variations are required to let them breathe the new 

 atmosphere. We have an instance in the above- 

 mentioned case of the blackbird. 



But there are also changes that arise quite gradually, 

 yet extinguish a species. When, for instance, a continent 

 sinks below the waves, however slowly, the land- 

 animals living on it will be destroyed. What happens 

 is, not that those animals are selected which are most 

 accustomed to the sea, but the animals squeeze into the 

 ever-decreasing territory, and when the time of the last 

 subsidence comes it is too short to transform them into 

 aquatic organisms. 



We see, therefore, that animal species are only 

 destroyed when sudden changes set in that make their 

 former habits impossible ; in other cases natural 

 selection is given time to modify them. 



The word " sudden " must not, of course, be taken 

 too strictly. When an animal has, in the course of 



