80 THE LESSON OF EVOLUTION 



migrations in all these cases were probably due to. 

 seeking out new supplies of food ; and it is this which 

 made Pterodactyles, birds, and bats take to flying. 



Early Vertebrates and the Use of Armour, Again, 

 why did so many early vertebrates the Ostraco- 

 dermi, the Dipnoi and the Labyrinthodonts acquire 

 such a strong armour? Surely not to defend them- 

 selves from their enemies ; for the Eurypterygians 

 were the only large animals living at the time, and 

 they do not appear to have been very formidable, as 

 they have no apparatus for catching a moderately 

 active prey. The principle of " following the food 

 supply," will, I think, give us an answer. They 

 may have fed upon the sea-weeds or upon the animals 

 living upon the sea-weeds of the sea-coast, and their 

 armour was to protect them from the rocks in the 

 rough and shallow waters which they frequented. 

 Possibly the ancestors of the amphibians would never 

 have reached the land if they had not been protected 

 by armour, especially by armour on the ventral 

 surface. 



Different Rates of Development. Another thing 

 we notice is that the rapidity of development of 

 different groups varies much. Among the verte- 

 brates some developed slowly at first, and then with 

 great rapidity (e.g. Mammals and Birds). Others 

 made a rapid advance, and then remained nearly 

 stationary (e.g. Ichthyosaurians, Dinosaurians, and 

 Pterodactyles). 



Again, in some cases a group, after varying much 

 at first, became almost extinct, but was preserved by 

 the persistence of, perhaps, a single genus, which 



