PEOPLING OF LAND AND SEA 183 



On the contrary, it has brought them closer together, in that in 

 both it contributes to their protection against chilling, since it 

 preserves a constant internal temperature in spite of the 

 variations in the external air ; that is to say, it renders them 

 warm-blooded animals endowed with a new independence of 

 external environment, and capable of resisting its modifications, 

 thus enabling them to achieve the highest organic develop- 

 ment of all living creatures. This achievement was reached, 

 however, by two different paths, not directly, but by a 

 combination of circumstances nowise working towards 

 that end. 



When the organisms of the warm-blooded Vertebrates 

 became accustomed to a constant temperature, this 

 temperature had to be artificially assured to the embryo, 

 which being inactive, could not itself produce it. This was 

 achieved passively in Mammals and actively in Birds, which 

 would thus seem in this respect to have made a definite 

 advance at some given moment. Those mammals which to-day 

 are least removed from the ancestral forms, the Monotremata, 

 represented by the two genera Omithorhynchus and Echidna, 

 lay their eggs in a sort of nest resembling a Bird's, and sit on 

 them in the same way. In the Marsupials, which come next in 

 the order of evolution, the eggs are no longer deposited. They 

 remain small and are retained in the womb of the mother, 

 where they develop without, as a rule, being in any way 

 linked with its walls. At birth the young are very small, and 

 their limbs are poorly developed. They are placed by the 

 mother in the ventral pouch, which most Marsupials possess, 

 and which contains the mammae, and here they develop. In 

 other Mammals the egg, detached early from the ovary, passes 

 into the uterus before it has exceeded in size the tenth of a 

 millimetre. It then contains only a very small amount of 

 reserve material. This, however, does not approximate it to 

 the small eggs of Fish, or even of Batrachians. It is not a 

 primitively small egg, but an egg which has reverted to a small 

 size, and which has hereditarily preserved the method of 

 development imposed upon large eggs by the enormous size 

 of their yolk. The reason for this mode of development, quite 

 obvious in the case of the large eggs, no longer obtains here, 

 and would be unintelligible and absurd if we did not know that 

 the primitive Mammals were oviparous and laid large eggs, 



