MAMMALIA 46 1 



One of the most noteworthy facts in connection with the 

 group is the degree of care given to the young by the parents, 

 especially the mother. This is true not merely in gestation but 

 after birth in the attendance of the mother to the needs of the 

 young, both in supplying food and in protecting from danger. 

 It must be remembered that this is done at the expense of the 

 parent's safety. This means that the species may be kept alive 

 by the birth of a smaller number of young, because more will 

 reach maturity than if left early to shift for themselves; and, 

 further, that a higher development of the young becomes possible 

 owing to the increased length of youth. The degree of develop- 

 ment at birth is quite variable. In a general way it is less in the 

 case of those whose parents can best protect the helpless young. 

 For example the young of the Camivora and of the Primates 

 are much less able to take care of themselves at birth than the 

 young of the Herbivora. Many biologists have called attention 

 to the fact that the greater care of the young implies higher 

 instincts and intelligence on the part of the parent. Any such 

 improvement is subject to the action of natural selection as an 

 advantageous characteristic. In turn a longer youth or period 

 of development is demanded for the maturing of these higher 

 instincts, thus in turn making a new demand on the parent for 

 more care and training. 



The social instinct is well represented among mammals. 

 This may vary from collection in mere shoals or herds where 

 food is abundant, to groups organized for offense and defense 

 and for work, — as wolves, deer, beavers, and men. Indiscrimi- 

 nate mating is the riile, yet in some instances strict monogamy is 

 found. In many cases mates are won by force, and this tends 

 to result in the selection and propagation of the strong. The 

 struggle among the males is accompanied by the development 

 in them of numerous structures which the females do not pos- 

 sess at all or at least in such degree: — as antlers, horns, tusks, 

 manes, — and greater size. 



Mammals do not have so great freedom of motion as the 

 birds, and hence do not make as much use of migration to escape 

 winter conditions. They tend to keep up an active life, like the 

 rabbit and wolf, or hibernate, like the woodchuck. There are some 



