84 THE MAMMALIA. 



their primitive, or rather to their most simple forms. For the 

 truly primitive forms from which the higher animals are descended 

 are as yet undiscovered — possibly, buried for ever beneath the 

 ocean, but ever sought with unwearied skill and patience. 



The labours of the embryologist and the comparative anato- 

 mist enable us, through the " scientific imagination," more or less 

 imperfectly, to reconstruct these ancient forms. But the oldest 

 mammals known, though almost reptilian in development in com- 

 parison with the mammalia of the present day, are yet highly 

 differentiated, and must have had long lines of ancestors. 



The problem to which I chiefly desire to attract attention in 

 this article is that of the extinction of species. It is usually 

 assumed that some catastrophe must have led to the destruction of 

 any given species. The glacial period accounts conveniently for 

 the disappearance of many forms. It is gravely stated that the 

 oceans are now not large enough for all the whales that formerly 

 flourished ; and a general drying up of swamps is supposed to 

 account for the disappearance of the earlier pachyderms. But we 

 have every reason to doubt a theory which requires constant 

 catastrophes to make it tenable. 



Another theory, which has an undoubted mingling of fact to 

 support it, is that the lower animals necessarily die out in the 

 struggle for existence before the higher ones. But, as I hope to 

 be able to show, though the lower orders of mammals tend to 

 disappear before the higher ones, yet species, and even orders, die 

 out where there has been no co?npetiiion with higher forms. 



It will make the difficulties of the question to be solved appear 

 more clearly if we go through the main orders of mammals, and 

 see how few species, comparatively speaking, survive, and how 

 mysterious are the laws governing their appearance and disappear- 

 ance. We will consider the Marsupials first. In this order, as is 

 well known, no placenta is developed, and the mother animal 

 transfers the embryo in a very immature condition to an external 

 pouch. Until lately, it was thought that the Marsupials made 

 their earliest appearance in Europe, and it was plausibly urged 

 that they had necessarily died out before the higher placental 

 forms. In the Miocene period^ they had become extinct, both in 

 Europe and in North America. 



