PEOPLING OF LAND AND SEA 185. 



placed on the breast. The position and number of the mammas 

 is thus seen to be independent of both diet and internal 

 organization, and simply depends on the degree of fecundity 

 and the posture. Heredity can intervene here as elsewhere 

 and seem to interfere with the conclusions that should follow 

 the application of these principles. But in reality it confirms 

 them when we discover that the supposed disharmony is 

 actually a concordance with primitive conditions of existence 

 now abandoned. The gait of the Ant-eaters, which progress 

 by supporting themselves on the edge of their feet or the back 

 of their toes, indicates clearly that these animals, though to-day 

 they burrow, were originally climbers, and for this reason 

 opposed the palms of their hands. On the other hand, the length 

 of their nails and their reproduction organization show that 

 in spite of the tremendous difference in the form of their heads, 

 they are related with the tree-Sloths ; they have preserved as 

 a fact their pectoral mammae. Elephants also have pectoral 

 mammae, and their past, once revealed, should explain the 

 reason for this arrangement characteristic of climbing 

 animals, which our knowledge of their present habits does not 

 suggest as a primitive condition, though the crossing of their 

 radius and ulna lends confirmation to it. 



We may thus sum up all that has just been said about the 

 origin of the land Vertebrates : — 



They are descended from the Batrachians, whose eggs- 

 became very large owing to the considerable accumulation of 

 this nutritive substance. The volume of these substances 

 caused the formation within the egg of a blastoderm over and 

 above the very limited portion of it destined to form the 

 embryo. The abundance of nutritive material permitted that 

 intensive embryogenetic acceleration which had already 

 suppressed the ancestral fins of the Batrachians to progress an 

 additional step, in consequence of which the imperfect 

 respiratory apparatus of the aquatic young gradually became 

 transitory, then simply suggested, to disappear completely 

 before the birth of the embryo. Thus, within the egg, the 

 embryo passed through its entire series of metamorphoses 

 under the protection of an amnion and an allantois — of purely 

 physiological origin, as we have seen. Henceforth the egg could 

 be deposited in the open air, protected as it was by a solid 

 shell. This stage of evolution, independently of the fate reserved 



