ii2 MODERN IDEAS OF EVOLUTION 



Equus, or EoJiippus and its successors in America are 

 links in a chain extending from the Eocene Tertiary 

 to the present time, can we suppose that by tracing 

 the same series further back it might include any 

 placcntal mammal ? We must answer, Decidedly 

 not, for if the whole time from the Eocene to the pre 

 sent has been required to produce the comparatively 

 small change from Palczotherium to horse, the same 

 rate in other cases would carry us back to the 

 Mesozoic period, long before we have any evidence 

 of the existence of placental mammals. In other 

 words, the Tertiary and Modern periods will give us 

 time enough only to effect changes of mammals within 

 the order Ungulata, and perhaps in only one section of 

 that order. The other orders must therefore consti 

 tute separate series, and these series must have been 

 advancing abreast of each other. Had each series a 

 separate origin, or is there any mammalian stock in 

 the Mesozoic from which, at the beginning of the 

 Tertiary, these several lines of types may have 

 diverged ? Here our information fails. We know 

 only small marsupial and insectivorous mammals in 

 the Mesozoic. On our hypothesis it is possible that 

 these may have been the progenitors of the more 

 varied and advanced marsupials and insectivora of 

 the Tertiary and Modern periods, but scarcely of the 

 placental mammals of the Eocene. There may have 

 been placental mammals, unknown to us, in the 

 Mesozoic, which may constitute the required stock. 



