96 THE LESSON OF EVOLUTION 



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 very much. Among the vertebrates 

 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). 

 The majority of invertebrates developed slowly and 

 steadily from the first to the present day (e.g. Echinoi- 

 dea, Pelecypoda, Gastropoda, Decapoda, and Insects) ; 

 but others rapidly attained a maximum, and then 

 declined (e.g. Brachiopoda, Trilobita, and Crinoidea). 

 It is the same with the extinction of groups. Most 

 perish slowly, as is illustrated by the gradual change 

 from Deutozoic to Mesozoic life ; but at the close of 

 the Cretaceous there was a rapid extinction of cepha- 

 lopoda and reptiles ; a fact very difficult to explain. 

 This rapid extinction seems to have occurred nearly 

 simultaneously all over the world in Europe, in 

 North America, in Australia, and in New Zealand. In 

 none of these countries do we find the Cretaceous 

 reptiles and cephalopods living with the Eocene birds 

 and mammals ; although such is said to have been the 

 case in Patagonia. 



We can only speculate on the causes. Perhaps 

 the cephalopods were killed off by the predaceous 



