38 Darwin, and after Darwin. 
or, according to our theory, in the work of tracing 
lines of pedigree. But now, the converse of this 
statement holds equally true. For it often happens 
that adaptive structures are required to change in 
different lines of descent in analogous ways, in order 
to meet analogous needs; and, when such is the case, 
the structures concerned have to assume more or 
less close resemblances to one another, even though 
\they have severally descended from quite different 
‘ancestors. The paddles of a whale, for instance, most 
strikingly resemble the fins of a fish as to their out- 
ward form and movements; yet, on the theory of 
descent, they must be held to have had a widely 
different parentage. Now, in all such cases where 
there is thus what is called an analogous (or adaptive) 
resemblance, as distinguished from what is called an 
homologous (or anatomical) resemblance—in all such 
cases it is observable that the similarities do not 
extend further into the structure of the parts than it 
is necessary that they should extend, in order that the 
structures should both perform the same functions. 
The whole anatomy of the paddles of a whale is quite 
unlike that of the fins of a fish—-being, in fact, that of 
the fore-limb of a mammal. The change, therefore, 
which the fore-limb has here undergone to suit it to 
the aquatic habits of this mammal, is no greater than 
was required for that purpose: the change has not, 
extended to any one feature of azatomical significance 
This, of course, is what we should expect on the 
theory of descent with modification of ancestral char- 
acters ; but on the theory of special creation it is not 
intelligible why there should always be so marked a 
distinction between resemblances as analogical or 
