4i 6 EVOLUTIONARY MORPHOLOGY 



the " age of reptiles," not only on account of the number of 

 different types which flourished, but also because of the 

 gigantic size to which many of them grew. 



In the Cretaceous, cold conditions returned with seasonal 

 variations. Mountain-building and glaciation occurred in 

 some parts of the earth, the temperature of which was now 

 considerably reduced. At this time and perhaps for this 

 reason the majority of the reptiles which had hitherto been so 

 successful, went extinct and were superseded by mammals as 

 the dominant animals. After this time, hot conditions set in 

 again for the main part of the Tertiary era, gradually diminish- 

 ing towards its close when a fresh bout of mountain-building 

 erected the Alps. Then followed the great Ice- Age. Mam- 

 mals continued evolving during this period, towards the end 

 of which man appeared. 



The most important changes in the environment as far as 

 the vertebrates were concerned were the drying-up of the 

 lagoons and estuaries in the Devonian, and the variations of 

 temperature. 



It is a characteristic feature of desiccated areas that the 

 water expanses which they possess shrink to ponds, and the 

 oxygen-content of the water decreases owing to the quantities 

 of decomposing organic matter with which the ponds become 

 filled. Under such circumstances it is obvious that fish which 

 are provided with means of supplementing their branchial 

 respiration would have a much greater chance of surviving, 

 and the first step in this direction was the habit of taking air 

 into the pharynx when at the surface. At the present day, 

 inhabitants of such waters show diverse adaptations, but by 

 far the most important of these from the present point of view 

 are the Dipnoi, with their lungs. There is little doubt that 

 the ancestors of the Tetrapods encountered and mastered 

 conditions of desiccation in fresh water, in the same way as 

 the modern Dipnoi. There is the further danger that under 

 these circumstances the water may dry up altogether, as it 

 does in the case of the swamps in which Protopterus lives, and 

 then the possession of a means for pulmonary respiration is 

 the only condition for survival. 



