164 CHILDHOOD OF ANIMALS 



more food than those of the higher mammals. In the latter there is 

 practically no food-yolk at all, and the eggs are microscopic in size, 

 being just visible to the naked eye. They soon develop a connection 

 with the maternal tissues which is a legacy from the blood-vessels 

 which spread out under the shell, and then replace that by a 

 new and more perfect means of drawing nutriment from the blood 

 of the mother, the structure which is known as the placenta. 



In mammals, therefore, the earliest stages of brood-care, instead 

 of being apparently conscious, external acts, which, so to say, might 

 be slurred over, bungled or forgotten, have become a part of the un- 

 conscious mechanism of the body. Instead of having to construct a 

 safe place in which to lay eggs, the mother retains them in the interior 

 of her body, supplies them with the necessary warmth and food, and 

 protects them from enemies at the peril of her own life. This change 

 from external to internal protection is least complete in the egg- 

 laying mammals, more complete in the kangaroos (where the young 

 are born when they are very small and placed in an external pouch 

 by the mother), and most complete in the higher mammals. Just 

 as there are some birds hatched when they are ready to run about 

 and others hatched when they are still blind and feeble, so there are 

 some mammals where the young are born almost ready to walk or 

 to run, and others where they are born blind and naked, differences 

 which depend on the habits of the animals and may be found amongst 

 species that are very closely allied. 



This internal organic brood-care is just as effective in protecting 

 the young as the brooding of birds, and it is followed by a still 

 longer period in which the new-born mammals are fed and guarded 

 by the mother. And so it happens that amongst mammals brood- 

 care is more elaborate and complete than in any of the other groups 

 of the animal kingdom. The young do not leave their parents until 

 they are well equipped to fight the battle of life for themselves. 

 The maintenance of the species by the production of enormous 

 families has ceased. Some of the little rodents may breed several 

 times in the course of the year and produce rather large litters, and 

 there are some fecund mammals, such ,as pigs, where the litter may 

 contain a dozen or even more. But these are rare exceptions. In 

 the vast majority of cases, mammals breed no more than once a 

 year, and in some instances only once in every two or three years. 

 The usual numbers are one, two or three at a birth, and the higher 

 in the scale of mammalian life one looks, the smaller is the number 

 that is usual. 



