186 PRIMITIVE FORMS OF LIFE 



for the skin, is common to Reptiles, Birds, and to the ancestral 

 Mammals represented to-day only by Monotremata. 



While this evolutionary stage is maintained among the 

 dry-skinned Reptiles and their descendants the Birds, a 

 regression takes place in the viviparous Mammals. The egg 

 ceases to accumulate abundant nutritive substances, but 

 reverts to complete segmentation, which these substances 

 would hinder. Heredity, nevertheless, conserves in the embryo 

 the method of development it acquired by reason of their 

 former abundance although that method has lost its original 

 purpose. The enveloping membranes then change their 

 function, and, while continuing to act as a protecting organ for 

 the embryo, also form a placenta that serves as the link between 

 mother and embryo. New causes now intervene, however, in 

 the formation of this placenta ; first, the irritation which 

 two vital membranes of different nature produce upon each 

 other when they are perpetually in contact, and secondly, the 

 exchange of various substances that takes place between these 

 two membranes or through their walls, since it is impossible 

 that the embryo should draw certain substances from its mother 

 without giving others in return. Did the case actually present 

 itself, we might see here an explanation of the supposed 

 influence which the first male exerts on the later conceptions of 

 the mother, a process called tclegony. Thus it is no t the first male 

 but his first offspring, who would be responsible for influencing 

 the mother. Nor must it be forgotten, moreover, that immature 

 eggs confronted with spermatozoa frequently assimilate 

 them, so that strictly speaking the spermatozoa may thus be 

 added to those substances capable of influencing the egg's 

 final evolution. 



Since the mammae arise as a result of the conservation of the 

 excretory functions of the skin, there are plenty of reasons to 

 account for a parallel evolution of the Mammals and Reptiles, 

 and for their having started to evolve at the same time ; 

 and to suggest that Mammals, at least the oviparous ones, 

 must be very early in origin. The case is otherwise with 

 Birds. They derive from a Reptilian type already highly 

 specialized, and also form a class whose homogeneity is in 

 marked contrast with the variety exhibited by Mammals. 

 Putting aside the development of feathers, a phenomenon of 

 a purely external order, the ancestor of the Bird must have 



