PEDIGREE OF MAMMALS 83 1 



tion, 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 rew^arded 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-re- 

 garding activities have been factors in evolution. 



Pedigree. — The origin of Mammals remains obscure, but there 

 is much to be said for their affihation to some ancient ReptiUan 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 con- 

 dition 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 the 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 

 and may share in the articulation for the lower jaw, that the quadrate is 

 often small, that there is a single temporal arcade comparable to the 

 mammalian zygomatic arch, that the teeth are heterodont, that the 

 pelvic bones unite in an os innominatum with a continuous ischiac 

 symphysis, that the scapula often has a spine, that the occipital condyle 

 may be double, that there is a beginning of reduction and consolidation 

 of skull bones, and so on. 



But it may quite well be that the Anomodontia are not in the direct 

 line of Mammalian ancestry, but represent a side-branch from transi- 

 tional forms connecting Reptiles and Mammals. 



The student should look back to the characters common to the 

 Amniota (Reptiles, Birds, and Mammals), e.g. the presence of amnion 

 and allantois. the absence of gills, etc., for these indicate a close alliance 

 far apart from Ichthyopsida, and it seems therefore unprofitable to look 

 for the roots of the Mammalian stock so low down as among Amphibians. 



