Chap. XIIL 
MORPHOLOGY. 
437 
moutli formed of many parts, consequently always have 
fewer legs; or conversely, those with many legs have 
simpler mouths ? Why should the sepals, petals, sta¬ 
mens, and pistils in any individual flower, though fltted 
for such widely different purposes, be all constructed 
on the same pattern ? 
On the theory of natural selection, we can satisfactorily 
answer these questions. In the vertebrata, we see a series 
of internal vertebras bearing certain processes and appen¬ 
dages ; in the articulata, we see the body divided into a 
series of segments, bearing external appendages; and in 
flow^ering plants, we see a series of successive spiral 
whorls of leaves. An indeflnite repetition of the same 
part or organ is the common characteristic (as Owen 
has observed) of all low or little-modified forms ; there¬ 
fore we may readily believe that the unknown progenitor 
of the vertebrata possessed many vertebrae ; the unknown 
progenitor of the articulata, many segments; and the 
unknown progenitor of flowering plants, many spiral 
whorls of leaves. We have formerly seen that parts many 
times repeated are eminently liable to vary in number 
and structure; consequently it is quite probable that 
natural selection, during a long-continued course of modi¬ 
fication, should have seized on a certain number of the 
primordially similar elements, many times repeated, and 
have adapted them to the most diverse purposes. And 
as the whole amount of modification will have been 
. effected by slight successive steps, we need not wonder 
at discovering in such parts or organs, a certain degree 
of fundamental resemblance, retained by the strong 
principle of inheritance. 
In the great class of molluscs, though we can homo- 
logise the parts of one species with those of other and 
distinct species, we can indicate but few serial homo¬ 
logies ; that is, we are seldom enabled to say that one 
