MODIFICATION OF REPRODUCTIVE PROCESS 77 



In these animals we find the modifications of the repro- 

 ductive processes to fit a purely terrestrial existence at 

 their highest level. Right down at the bottom of the 

 scale of mammals we find two very ancient creatures 

 still surviving in Australia and New Guinea (Echidna and 

 Ornithorhynchus) in which the eggs are large, provided 

 with a supply of yolk as in the case of Reptiles and 

 Birds, and are actually laid in the case of Ornitho- 

 rhynchus being deposited in an underground burrow, 

 while in that of Echidna they are carried about by the 

 mother in a specially developed pouch on the lower 

 surface of the body. 



In the next phase of evolution, represented by the 

 Kangaroos and other Australian mammals and by the 

 American Opossums, the eggs are hatched while still 

 inside the mother's body, so that instead of eggs being 

 laid, young are born. These young are very helpless and 

 inperfectly developed, so the mother carries them about 

 for some time, either hanging on to her body or contained 

 in a pouch specially developed for the purpose, until they 

 are able to look after themselves. 



In the ordinary mammals a step further has been 

 taken, the young animal being retained within the 

 mother's body for a much longer period, until a much 

 higher degree of development has been reached. In this 

 way not only are the early stages which should be aquatic 

 spared the need of a watery environment but they remain 

 within the body and, as it were, part of the body of a 

 fully developed adult creature, with all its capacity 

 developed for looking after itself in the struggle for 

 existence. 



Here again we find very interesting arrangements 

 adapting the embryo to its life within the body of the 



