90 THE WHENCE AND THE WHITHER OF MAN 



eggs of a worm. Moreover, where the individuals are 

 longer lived and the generations follow one another at 

 longer intervals, the number of favorable variations 

 and the possibility of conformity to environment 

 through these is greatly lessened. In such a group it 

 is of the utmost importance that every egg should de- 

 velop ; the destruction of a single one is a real and im- 

 portant loss to the species. It is not enough to pro- 

 duce such an egg; it must be most scrupulously 

 guarded. Even the egg of the platypus is deposited in 

 a nest in a hole in the bank, and the female Echidna 

 carries the egg in a marsupial pouch until it develops. 

 Notice further that among certain species of fish, 

 amphibia, and reptiles, the females carry the eggs in the 

 body until the embrj^os or young are fairly developed. 

 Viviparous forms are unknown by birds, probably be- 

 cause this mode of development is incompatible with 

 flight, their dominant characteristic. Putting these 

 facts together, what more probable than that certain 

 primitive egg-laying mammals sliould have carried the 

 eggs as long as possible in the uterus. The embryo 

 imder these conditions would be better nourished by a 

 secretion of the uterine glands than by a very large 

 amount of yolk. The yolk would diminish and the egg 

 decrease in size, and thus the marsupial mode of devel- 

 opment would have resulted. And, given the marsupial 

 mode of development and an embryo possessing an al- 

 lantois, it is almost a physiological necessity that in 

 some forms at least a placenta should develop. That 

 the placenta has resulted from some such process of 

 evolution is proven by its different stages of develop- 

 ment in different orders of mammals. And even the 

 feeblest attachment of the allantois of the embryo to 



