298 MAMMALIA. 



stage of development at which the young are born. It is longest 

 in the large terrestrial, and the colossal aquatic animals (Unyulata, 

 Cetacea), which live under favourable conditions of nourishment. 

 The young of these animals are so far advanced in their bodily 

 development at birth, that they are ;ible to follow the mother (to 

 a certain extent like jwcecoces). The period of gestation is relatively 

 shorter in the Curn-irura, the young of which are born naked and 

 with closed eyes and, like altrices, are for a long time completely 

 helpless, and need the care and protection of the mother. It is, 

 however, shortest in the aplacental Monotremes"' ;: and Marsupials. 

 In these animals the young, which are born at a very early stage (in 

 the Kangaroo they are no larger than a nut), pass into a pouch 

 formed by cutaneous folds in the inguinal region, and here adhere 

 firmly to the nipples of the mammary glands. In this pouch, as in a 

 second uterus, they are nourished by the secretion of the mammary 

 glands, which assume at this early stage the nutrient function of the 

 absent placenta. The number of the young, which are born, also 

 varies very greatly in the different genera. The large Mammalia, 

 of which the period of gestation is longer than six months, as a rule 

 bear only one, more rarely two young ; but in the smaller Mammals 

 and some domestic animals (Pig) the number is considerably larger, 

 so that twelve to sixteen, or even twenty young may be born at one 

 time. The number of teats on the mother usually indicates the greater 

 or smaller number of the progeny. 



Many Mammals live a solitary life, and pair only at the breeding 

 time ; they are principally such carnivorous animals as find their 

 subsistence by hunting in definite hunting grounds, like the Mole 

 in its subterranean passages. Others live united in companies, in 

 which the oldest and strongest males frequently undertake the care 

 of protection and leadership. Most Mammals seek their food by day. 

 Some, e.g., the Bat, leave their hiding places in the twilight and at 

 night. Most Carnivora and numerous Ungulata also sleep in the 

 daytime. Some Rodentta, Insectivora, and Carnivora fall, during 

 the cold season of the year when food is scarce, into an interrupted 

 (Bear, Badger, Bat), or continuous (Dormouse, Hedgehog, Marmots) 

 winter sleep in their hiding places which are often carefully protected, 

 or in nests formed in the earth. During this time the temperature 

 is lowered, the respiration is less active, the heart-beat is slowed, and 

 they take up no food, but consume the fat masses which were stored 

 up in the autumn. The following nnimals are known to migrate : 



* Virlf note on p. 2'.H',. 



