1898) JMPERFECTION 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. 
Not 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 
North 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 North 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 
