chap, viii.] MESOZOIC MAMMALIA. 159 



young born each year in the world are probably as some millions 

 to one ; and it is very easily conceivable that in a thousand 

 years the former might, under pressure of rapidly changing con- 

 ditions, become modified into a distinct species, while the latter, 

 not offering enough favourable variations to effect a suitable 

 adaptation, would become extinct. We must also remember 

 the extreme specialization of many of the large animals that 

 have become extinct — a specialization which would necessarily 

 render modification in any new direction difficult, since the in- 

 herited tendency of variation would probably be to increase the 

 specialization in the same directions which had heretofore been 

 beneficial. If to these two causes we add the difficulty of 

 obtaining sufficient food for such large animals, and perhaps the 

 injurious effects of changes of climate, we shall not find it diffi- 

 cult to understand how such a vast physical revolution as the 

 Glacial epoch, with its attendant phenomena of elevations and 

 subsidences, icy winds, and sudden floods by the bursting of lake 

 barriers, might have led to the total extinction of a vast number 

 of the most bulky forms of mammalia, while the less bulky 

 were able to survive, either by greater hardiness of constitution 

 or bv becoming more or less modified. The result is apparent in 

 trie comparatively small or moderate size of the species consti- 

 tuting the temperate fauna, in all parts of the globe. 



It is much to be regretted that no mammalian remains of earlier 

 date have been found in Australia, as we should then see if it is 

 really the case that marsupials have always formed its highest 

 type of mammalian life. At present its fossil fauna is chiefly 

 interesting to the zoologist, but throws little light on the past 

 relations of this isolated country with other parts of the globe. 



Mammalian remains in the Secondary Formations. 



In the oldest Tertiary beds of Europe and North America, we 

 have (even with our present imperfect record) a rich and varied 

 mammalian fauna. As compared with our living or recent highly 

 specialized forms, it may be said to consist of generalised types ; 

 but as compared with any primeval mammalian type, it must be 

 pronounced highly specialized. Not only are such diversified 



