196 THE CONTROL OF LIFE 



instincts, so intense in their expression, are sharply 

 punctuated, being asleep for most of the year ; the 

 mother is not encumbered by having to carry the un- 

 born young ; the egg-laying is physiologically inexpen- 

 sive; the males often share in nest-making and nur- 

 turing ; there is sometimes prolonged monogamous 

 partnership and what at any rate mimics human affec- 

 tion ; the care of the eggs and of the young is so suc- 

 cessful that the greatly economised reproduction works 

 well ; the young are in some cases able to look after 

 themselves soon after hatching. 



The three lowest Mammals (the duckmole and the 

 spiny ant-eaters) lay eggs ; the Marsupials, like opos- 

 sum and kangaroo, have but a short gestation, and 

 only one of them is known to have a true placenta* 

 binding the unborn young to the mother's womb. The 

 true placenta, which mediates in the intimate ante- 

 natal partnership, is confined to the ordinary mam- 

 mals, Hke carnivores, ungulates, rodents, insectivores, 

 monkeys, and man, and it permits, in a new way, of a 

 high degree of development being reached before the 

 young are bom. In elephants the ante-natal life or 

 period of gestation actually lasts for more than a year. 



The typically mammahan method of motherhood 

 may cost the mother a good deal, but she often gets 

 something back in increased health, and it is probably 

 on the whole less costly than the spawning method. It 

 has the advantage of giving the offspring a fine send- 

 off with a big educable brain. It is probable that the 

 long partnership between mother and offspring before 



