REPTILES AND AMPHIBIANS 133 



But it seems very questionable if there really are 

 these different types of animals. Even the persistent 

 types have been at one time evolved from lower 

 organisms, and were therefore once plastic ; their 

 persistence is not an eternal and unchangeable feature. 

 That many animals have remained unchanged for very 

 long periods is clearly due to the fact that this form 

 was the most suitable for the environment in which 

 they lived. We must not forget that it is not absolutely 

 necessary that all the individuals in a species shall be 

 modified so as to produce a new one. Let us take the 

 case of a species of aquatic animals in a pond. The 

 scarcity of food caused by their multiplication will put 

 those in a more favourable position that can travel on 

 to the land and adopt a different diet. But if a large 

 number of the animals leave the water, there is no 

 longer any need for the others to become land-animals, 

 as they have now plenty of room and food. Natural 

 selection means pressure ; new species are only 

 formed when they must change in order to avoid 

 destruction. 



This is quite clear when we consider the origin of 

 migratory birds. Here it was the migration that created 

 the species, the animals wandering into new regions for 

 which they needed new characters. Migration is an 

 important principle in the formation of species generally. 

 Thus the flying insects that reached certain small 

 islands lost their wings and became a new species, 

 because in this case natural selection always favoured 

 the worst fliers ; the good fliers were the first to tumble 

 into the water. Animals usually only rise to more 



