ANCESTRY OF HOMOIOTHERMIC ANIMALS 15 



Environment during Evolution of Homoiotherms 



Vertebrates had their origin in the water and some 

 of them have slowly spread into land habitats. The 

 earliest vertebrates already had a long heritage from 

 the past. Even their blood had a somewhat distant rela- 

 tion to sea water, which probably served their remote 

 ancestors as a body flnid. Types which attained to life 

 on land were obliged to develop resistance to loss of 

 water and to endnre great changes in temperature. One 

 of the means of gaining relief from the continual and 

 marked changes of land climates was the attainment of 

 the homoiothermic condition. In other words, the ability 

 to maintain a constant temperature which permits unin- 

 terrupted metabolic activity at approximately optimum 

 rate is an adaptation to life on land, where climate is 

 variable. 



The earliest birds and mammals were carnivores and 

 insectivores. Homoiotherms and seed plants evolved to- 

 gether and Berry (1920) believes that the evolution of 

 the former was to be to a considerable degree dependent 

 on that of the latter. The development of seeds and 

 fruits furnished nourishment which was in concentrated 

 form and which was available at all seasons of the 

 changeful year. The land animals which took advantage 

 of the newly developed seeds and fruits as food were 

 thus relieved to a considerable degree of the necessity 

 for continually seeking and capturing living prey. This 

 surety of food supply made easier the maintenance of 

 metabolism at a continuously high rate. ^*It may be sug- 

 gested that the changing food supply which is due to 

 the evolution of the flowering plants, and which is sug- 

 gested as one of the important factors in the evolution 

 of the higher mammals, was also one of the factors that 



