126 SEX-LORE 



her eggs, the mother retains them in her body, supply- 

 ing the developing embryos with warmth and sus- 

 tenance up to birth. This insures greater safety 

 for them. After birth, instead of having to seek food 

 for the young, she is fitted with special milk glands 

 yielding nourishing food from her own body, which 

 sustains the young until they are old enough to take 

 other food. This, again, is of great advantage, as 

 there is thus no danger of the young starving. At the 

 same time, the bond between mother and offspring 

 becomes very much closer, as with the dependence 

 of the young the affection of the mother for them 

 increases. It has been pointed out repeatedly that, 

 the more developed an animal, the longer it takes 

 to mature; it is, therefore, not surprising to discover 

 that mammals, with rare exceptions, are not prolific. 

 In the vast majority of cases they breed once or twice 

 a year, rarely oftener; and the usual number of off- 

 spring at birth is one, two, or three, although it may 

 go up to six and even more in certain mammals. 



While the production of living young is character- 

 istic of the mammals, still we find some exceptional 

 instances of viviparous species already below the 

 mammals, namely, among the fishes and reptiles. 

 On the other hand, we have seen that the lowest 

 mammals, the duck-billed platypus and the ant- 

 eater, lay eggs, though they have other contrivances 

 which relate them to the mammals. As usual, here 

 too Nature makes no jumps, but works by transition 

 stages. The lowest lass of mammals producing 

 living young are the marsupials. The marsupial egg 

 is hatched within the mother's womb; the young, 

 however, is born in a very immature condition, as 



