The geographical distribution of animals, with a study of the relations ...

chap, viii.] MESOZOIC MAMMALIA. 159young born each year in the world are probably as some millionsto one ; and it is very easily conceivable that in a thousandyears the former might, under pressure of rapidly changing conditions,become modified into a distinct species, while the latter,not offering enough favourable variations to effect a suitableadaptation, would become extinct. We must also rememberthe extreme specialization of many of the large animals thathave become extinct—a specializationwhich would necessarilyrender modification in any new direction difficult, since the inheritedtendency of variation would probably be to increase thespecialization in the same directionswhich had heretofore beenbeneficial. If to these two causes we add the difficulty ofobtaining sufficient food for such large animals, and perhaps theinjurious effects of changes of climate, we shall not find it difficultto understand how such a vast physical revolution as theGlacial epoch, with its attendant phenomena of elevations andsubsidences, icy winds, and sudden floods by the bursting of lakebarriers, might have led to the total extinction of a vast numberof the most bulky forms of mammalia, while the less bulkywere able to survive, eitherby greater hardiness of constitutionor bv becoming more or less modified. The result is apparent intrie comparatively small or moderate size of the species constitutingthe temperate fauna, in all parts of the globe.It is much to be regretted that no mammalian remains of earlierdate have been found in Australia, as we should then see ifit isreally the case that marsupials have always formed its highesttype of mammalian life. At present its fossil fauna is chieflyinteresting to the zoologist, but throws little light on the pastrelations of this isolated country with other parts of the globe.Mammalian remains inthe Secondary Formations.In the oldest Tertiary beds of Europe and North America, wehave (even with our present imperfect record) a rich and variedmammalian fauna. As compared with our living or recent highlyspecialized forms, it may be said to consist of generalised types ;but as compared with any primeval mammalian type, it must bepronounced highly specialized. Not only are such diversified