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 offermg 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 
harriers, 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 by becoming more or less modified. The result is apparent in 
the 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 
