1898] IMPERFECTION OF THE GEOLOGICAL RECORD 331 



Presumably, therefore, if the marine mammals are derived from 

 terrestrial quadrupeds, as seems probable, we ought to find fossil 

 records of their partially evolved ancestors in some of these de- 

 posits. As a matter of fact, we have hitherto found nothing. 

 The earliest known Cetaceans and Sirenians are more nearly like 

 normal land-mammals than the later and existing genera of the 

 same orders ; but the approximation is only very slight. They 

 are completely differentiated on their earliest appearance, and the 

 geological record, so far as explored, affords no clue whatever to 

 their origin and affinities. It is, of course, possible that these 

 aquatic animals originated during the Mesozoic period in some 

 land-locked sheet of water or lake, of which the sediments have 

 been destroyed or not yet discovered. Some American palaeontol- 

 ogists think it very probable that the seals originated in this way 

 at an early Tertiary period in North America, where there were 

 already great lakes. It may therefore be that the history of the 

 other marine mammals is similar. 



Xot only is our ignorance deep and absolute in respect to many 

 of these most fundamental problems : it also progresses very slowly 

 even in some instances where enlightenment begins. Consider the 

 case of the ancestral birds. Of the all-important Archaeopteryx we 

 still have only two good specimens from one formation and locality; 

 and we know nothing more of the great race to which it belongs. 

 Of the Cretaceous toothed birds, which have now been known for 

 more than a quarter of a century, the only satisfactory specimens 

 hitherto discovered are a few from one formation in one region of 

 Xorth America. 



Again, our knowledge of the history of the elephants has scarcely 

 progressed (except in minute details) since Falconer left the subject 

 at the time when Darwin first referred to it. They can be traced 

 back to a certain point ' in the Miocene period, where Dinotherium 

 seems to be an ancestor of the order in the Old World ; but there 

 our genealogy stops. Of Dinotherium itself we know very little 

 accurately beyond the teeth ; while of its origin and ancestry we 

 can still not recognise a trace among the mammals of earlier date. 



Within the last quarter of a century enormous progress has 

 indeed been made in discovering links in the chain of life and in 

 determining the facts of distribution at different periods. The 

 working out of the Tertiary mammals in Xorth America, for 

 example, has opened up a new era in Biology and Geology. But 

 most of the animals discovered and named are known only by a 

 few fragments, which do not reveal even a tolerably complete 

 skeleton. There is very little material for detailed comparison ; 

 and only in a few instances is it possible to study individual and 

 local variations. There are very few even of the best known species 



