4 INTRODUCTORY 
protection the mother, in a large number of cases, has a special 
pouch enclosing the mamme, 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, they 
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 Buffalos, 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, 
