THE EVOLUTION OF THE MAMMALIA. 25 
drawbacks. It is only by a combination of all of them that definite 
and final results are to be attained. 
Comparative anatomy lies at the very foundation of the whole 
zoological structure, and an accurate knowledge of it is an indis- 
pensable prerequisite to any intelligent use of the other methods. 
Unaided and alone this method has achieved many notable tri- 
umphs in the demonstration of the mutual relationships of ani- 
mals, and in the hands of the great masters it has often seemed 
to be an all-sufficient means of inquiry. But not every one can 
bend the bow of Ulysses, and, even under the most favorable 
conditions, finality cannot be reached by this method. It suffers 
from the very important defect that it possesses no sure and cer- 
tain criterion by which to distinguish between the similarities that 
result from actual relationship, due to a common descent, and 
those which have been independently acquired (parallel or con- 
vergent development). It thus fails to determine with certainty 
the taxonomic value of a given likeness or unlikeness in structure. 
A very common fallacy in zoological reasoning is the assumption 
that, because a number of allied animals display a certain struc- 
ture, their common ancestor must also have possessed it and 
have transmitted it to the diverging groups of descendants. 
This may have been the case, but it is almost as likely not to 
have been, because the structure in question may have been inde- 
pendently acquired many times. For example, if we compare the 
skeleton of the horse and the camel, representing two distinct 
orders of hoofed animals, we find that they agree in a number of 
details in which each differs from its nearer allies, the members of 
its own order. What is the significance of those resemblances ? 
Were they found in the far distant common ancestor of horses 
and camels ? and have they been lost and suppressed in other 
hoofed animals ? or, on the other hand, were these likenesses 
of detail independently acquired in the evolution of the two groups ? 
The comparative method alone would not render any certain 
answer to these questions, and, in fact, the only sure method of 
answering them would be to study the extinct members of the series 
which have led to the existing camels and horses. A study of 
