HISTORY OF MAMMALS. 679 



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. 



History of Mammals. — As to the origin of Mammals we can only 

 speculate. There are some remarkable resemblances between Mono- 

 tremes and certain extinct Reptilian types, known as Anomodontia or 

 Theromorpha, and these again exhibit affinities with the extinct Laby- 

 rinthodont Amphibians. Amphibians and Mammals agree in having 

 two occipital condyles, small quadrates, large squamosals, and in certain 

 characteristics of pectoral and pelvic girdles. Possibly the ancestral 

 Mammals and the Theromorph Reptiles diverged from a common 

 Amphibian stock. 



The oldest known remains of Mammals aie some fossils from Triassic 

 locks, and similar types have been found in Cretaceous and Jurassic 

 beds ; most of these Mesozoic fossils are but small pieces of small 

 animals, and secure conclusions as to their nature are not readily 

 reached. The earliest suggest affinities with Marsupials and Insecti- 

 vora. Many of the Mesozoic Mammals belong to a group which has 

 received the name of Multituberculata, on account of the longitudinal 

 rows of tubercules on the back teeth. It is probable that these forms, 



