GENERAL LIFE OF MAMMALS. 737 
The duckmole lays eggs and brings up her young in the 
shelter of the burrow; the Echidna has a temporary pouch. 
In Marsupials the time of gestation is very short, and there 
is rarely a true placental union between the unborn young 
andthe mother. The new-born Marsupials are very helpless, 
and are in most cases transferred to an external pouch or 
marsupium, within which they are nurtured. In Eutherian 
Mammals the gestation usually lasts much longer than in 
Marsupials,—its duration varying to some extent with the 
rank in the mammalian series ; but there are great differences 
in the condition of the young at birth. “In those forms,” 
Sir W. H. Flower says, ‘‘ 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 Ungulates or Hoofed Mammals, where in the 
majority of cases defence from foes depends upon fleetness 
of foot, or upon hugé 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 aninfals pass their 
existence.” 
The maternal sacrifice involved in the placental union 
between the mother and her “foetal parasite,” in the pro- 
longed gestation, in the nourishment of the young on 
milk, and in the frequently brave defence of the young 
against attack, has been rewarded in the success of the 
mammalian race, and has been justified in the course of 
natural selection. But it is important to recognise that the 
maternal sacrifice—whatever its origin may have been— 
expresses a subordination of self-preserving to species- 
maintaining. Thus other-regarding as well as self-regarding 
activities have been factors in evolution. 
Pedigree.—The origin of Mammals remains obscure, but there 
is much to be said for their affiliation to some ancient Reptilian stock, 
such as the Anomodontia (especially the Theriodontia). 
In several features the Monotremes link the Mammals to living 
Reptiles, ¢.g. the structure of the pectoral girdle, the cloaca, the 
condition of the genital ducts, the relatively large ova with meroblastic 
segmentation, but it is out of the question to think of any cf the 
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