532 MAMMALS. 



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 importance of prolonged infancy, as illustrated among 

 monkeys, should be recognised in connection with the evolu- 

 tion of gentleness. 



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 ends, illustrates that altruistic as well as egoistic 

 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 Monotremes and certain extinct Reptilian types, 

 known as Anomodontia or Theromorpha, and these again 

 exhibit affinities with the extinct Labyrinthodont Amphi- 

 bians. Amphibians and Mammals agree in having two 

 occipital condyles, small quadrates, large squamosals, and in 

 certain characteristics of pectoral and pelvic girdles. Pos- 

 sibly the ancestral Mammals and the Anomodont Reptiles 

 diverged from a common Amphibian stock. 



The oldest known remains of Mammals are some fossils 

 from Triassic rocks, 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. It is likely 

 that future discoveries will make the record much less frag- 

 mentary. Many of the Mesozoic mammals belong to a 

 group which has received the name of Multituberculata, 

 on account of the longitudinal rows of tubercles on the 

 back teeth. It is possible that these forms, e.g., Plagiau- 

 lax, Tritylodon, Polymastodon, should be ranked beside the 

 Monotremes. 



Other Mesozoic forms, such as Dromatherium, Triconodotir 



