3 o THE INTERNATIONAL MONTHLY. 
may have had, as a matter of fact, an entirely different ancestry 
and have migrated half-way around the globe to the place where 
it is found. The sudden appearance of an entirely new type in 
regions where no hint of an ancestry for them can be discovered, 
is usually to be explained by a migration of this sort. 
The reader might be easily led to infer from this discouraging 
account of the difficulties and limitations of palaeontology, that 
that science could afford no real help in the solution of genea- 
logical problems, but any such inference would be altogether 
unjustified, for the other side of the case has yet to be heard. 
Several of the limitations are but partial, not applying to particu- 
lar cases, while others are difficulties that are slowly yielding to 
continued investigation and the exploration of new regions. The 
opening up, geologically speaking, of our own West has added 
wonderfully to our knowledge of life in the earth's past history, 
and the exploration of South America, even now in progress, is 
bringing to light a still greater wealth of new and valuable mate- 
rial. When the whole earth is as well known geologically as 
Western Europe, the palaeontological record, it may be confi- 
dently expected, will prove to be of inestimable value to the 
zoologist. 
As we have seen already, there is no prospect of our ever being 
able to reconstruct an unbroken life-history of the earth, but, on 
the other hand, certain chapters of that history have been pre- 
served in an astonishing degree of fullness and detail. For ex- 
ample, the series of fresh-water deposits in the Western United 
States extends with hardly a break from the lower Eocene into 
the Pliocene, and every stage has yielded an abundance of well 
preserved fossil vertebrates, especially of mammals. Under such 
conditions, palaeontology becomes a most useful supplement to 
comparative anatomy and embryology. Its preeminent advan- 
tages are as follows : — 
In the first place, palaeontology gives us, in many cases, the 
members of the actual genealogical series, and in the true order of 
their succession in time. In many groups of animals such series 
have already been recovered, so full, so complete, that no observer 
