XXI 



EVOLUTION OF PLACENTAL MAMMALS AND ITS 



RELATION TO THE CLIMATIC AND 

 GEOGRAPHICAL HISTORY OF THE CENOZOIC 



1 . Eutherians at the end of the Mesozoic 



Several different lines of evidence converge to show that all the 

 eutherians (placentals) have been derived from small, perhaps noc- 

 turnal, insectivorous or omnivorous animals, living in the Cretaceous 

 period about ioo million years ago. Many features of marsupials 

 and placentals alike suggest origin from a small Cretaceous shrew-like 

 form, perhaps itself descended from some animal like the Jurassic 

 pantotheres. It is especially interesting, therefore, that fossil evidence 

 is now available to show that both opossums (p. 563) and placental 

 insectivores (p. 583) existed in the Cretaceous. We may be reasonably 

 sure that the population from which those groups were derived 

 resembled both of these animals, which are indeed basically similar. 

 At this Cretaceous period the arrangements for nourishing the young 

 were presumably not yet fully developed and in the marsupials 

 (Metatheria) they have remained at a simple level, little above ovo- 

 viviparity, though the condition in Perameles makes it doubtful 

 whether an allantoic placenta has been lost by the other marsupials. 

 The stock that was to give rise to the eutherians was therefore 

 already differentiated in the Cretaceous; as the revolution proceeded 

 these animals began to flourish and to develop into several divergent 

 populations. The only placental types known to have lived during 

 Cretaceous times were insectivores; yet by the very beginning of the 

 Cenozoic period, in the formations known as the Palaeocene, several 

 different types of placental are found. 



2. The end of the Mesozoic 



It is not easy to discover any close connexion between the climatic 

 changes and this early flowering of the placentals. Throughout the 

 period of the Cretaceous earth movements the land gradually became 

 higher and the climate probably colder, at least in some parts of the 

 world. Indeed, this process had been going on intermittently through- 

 out the Mesozoic. In the Permian there was a series of glaciations even 



