PEOPLING OF LAND AND SEA 181 



accelerated. It passes rapidly through the patrogenetic phases 

 of its evolution that had as their end the hereditary formation of 

 organs such as the branchiae, which it now has no need to 

 develop and can resorb as soon as they appear. Before leaving 

 its egg-shell the embryo can await the acquisition of sufficient 

 vigour to enable it to seek its own nourishment and to win 

 victory over those untoward chances to which existence in 

 the open air might expose it. The mother is no longer obliged 

 to return to the water in order to obey the hereditary instinct 

 of an aquatic organism which still exercises influence upon her 

 progeny. Independence of a humid environment, together with 

 an air-breathing mode of life, is finally acquired for the 

 Vertebrates. It is the life that will be led by Reptiles, Birds, 

 and Mammals, and which will make them masters of the world. 

 Since the respiratory system was at first feeble, and the 

 arrangement of the circulatory system a survival of the time when 

 breathing was done by gills, the internal combustion in the body 

 of the Vertebrate was not capable of producing a body-heat 

 that could defy the variations in the external temperature, 

 for the oxygen with which the blood plasma was only very 

 incompletely saturated did not suffice to supply the organs 

 with all they could have consumed. The creature lived in 

 a state of dependence on these conditions. Its internal 

 temperature, which regulated its activity, varied according to 

 that outside. When the latter fell below or rose above certain 

 limits the vital functions either decreased or ceased altogether. 

 The animal's vitality thus became intermittent. This is the 

 case with Reptiles. However, in their own way, and by different 

 methods, both Birds and Mammals succeeded in bringing their 

 lungs to a high degree of perfection, and in completely separating 

 in the heart and circulatory system the blood charged with car- 

 bon dioxide from the blood saturated with oxygen, thus assuring 

 a full supply of oxygen to the structural cells and organs, to 

 whose existence and functioning it is essential. This functioning, 

 if it consumes energy, develops a proportionate quantity of 

 heat, both the energy and the heat resulting from the com- 

 bustion of foodstuffs. Moreover, the heat developed is retained 

 within the organism once it has become warm, by the layers 

 of air imprisoned between the feathers of birds and in the fur 

 of Mammals. Certainly neither feathers nor hairs were 

 instituted for such a purpose ; these tegumentary growths 



