GENERAL LIFE OF MAMMALS. 713 



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 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." 



The maternal sacrifice involved in the placental union 

 between the mother and her "fcetal 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. 



The origin of Mammals remains obscure, but there is much to be said 

 lor 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, e.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 of the living 

 types of Reptiles as near the direct line of Mammalian pedigree. 



In Anomodontia, there are so many mammalian features in the 

 skeleton that in spite of the complex lower jaw, articulating with a 

 fixed quadrate, the presence of an os transversum, pre- and post- 

 frontals, etc., some have doubted whether they should be ranked as 

 Reptiles at all. We may note that they were purely terrestrial 

 animals (of large size) with limbs lifting the body high off the ground, 

 that the squamosal sometimes descends far down outside the quadrate 



