INTRODUCTORY 



protection the mother, in a large number of cases, has a special 

 pouch enclosing the mammae, into which the young are transferred 

 at birth, and in which they remain till they are well developed. 

 Among the higher, or Placental types, however, where a connection 

 exists between the maternal and foetal circulations previous to birth, 

 the young are always born in a much more highly developed state 

 than among the Marsupials, although we meet with great variations 

 in this respect. In those forms which habitually live in holes, like 

 many Rodents, the young are always very helpless at birth ; and 

 the same is also true of many of the Carnivora, which are well able 

 to defend their young from attack. In the great order of 

 Ungulate, or Hoofed Mammals, where in the majority of cases 

 defence from foes depends upon fleetness of foot, or upon huge 

 corporeal bulk, the young are born in a very highly developed 

 condition, and are able almost at once to run by the side of the 

 parent. This state of relative maturity at birth reaches its highest 

 development in the Cetacea, where it is evidently, associated with 

 the peculiar conditions under which these animals pass their 

 existence. In the Primates, however, we again find the young 

 produced in a more or less helpless condition, and requiring a long- 

 period before they attain their full development, this being more 

 especially the case with those higher forms which approximate in 

 structure to man. 



In point of size mammals vary to a greater extent than the 

 existing members of any one class of animals, and include the 

 largest living inhabitants of the earth. The extremes of size are 

 marked on the one hand by the whale known as Sibbald's Rorqual, 

 which attains a length of eighty feet and a weight of nearly as many 

 tons, and on the other by the Pigmy-Shrew and the minute Harvest- 

 mouse, which can climb a stem of wheat. 



Of all the living creatures inhabiting our globe, mammals are by 

 far the most important in their economic uses, since, in addition to 

 being the only animals capable of labour for human benefit, the}' 

 furnish the greater portion of the animal food of many races of man, 

 and likewise a large amount of their clothing. In these respects 

 the Ungulates hold the first place. 



As regards employment for labour, with the exception of the 

 Dogs used for sleighing by the Esquimaux, and those which among 

 some European nations draw light carts, all the mammals in general 

 use are Ungulates. Of the first importance are the Horses and 

 Asses, which are employed as beasts of draught or burden over 

 nearly the whole globe. Among many nations, however, cattle, as 

 represented by the true Oxen, the Buftalos, and the Yaks of Tibet, 

 occupy a still more important position, while in the highlands of 

 Tibet Sheep are largely used for carrying burdens. In other regions, 

 again, the place of the Horse and the Ass is taken by the Camels, 



