102 
THE PRINCIPLES OF BIOLOGY. 
undergone many changes. We have here also an explanation 
of the fact that the heart and other internal organs are more 
directly developed than the external organs, which are most 
exposed to the modifying actions of external forces. Thus all 
the remarkable facts of embryology are brought into harmony 
with the principles of the theory of evolution. 
Chapter YI. 
.If we turn from the organisms in the stage of develop¬ 
ment to those which have already arrived at the stage of 
maturity, we find that sub-groups belonging to the same 
group as a rule are similar in their structure. Let us take 
the vertebrate animals as an example; there are the mammals, 
the birds, reptiles, fishes, and amphibia, all with the same 
structure of the vertebral column, ribs, and four limbs. The 
latter are not always directly recognisable, and in some cases are 
entirely wanting, but as a rule we can trace their rudiments 
or homologous structures. In the class insecta, which Mr. 
Spencer quotes, there is the uniform structure of twenty 
segments, which is also present in the crustaceans. Here 
the case is still more remarkable, as the number is always 
the same. Although sometimes they are difficult to make 
out, several segments having apparently united to form one, 
their presence can always be demonstrated. There are 
numerous similar examples in the animal and vegetable 
kingdoms. 
Now, it is evidently absurd to suppose that such facts are 
the result of design of the Creator, or of chance, or of neces¬ 
sity ; but if we assume that the present organic forms have 
arisen by modification and divergence from common ancestral 
stocks, we at once understand these similarities and analogies 
as a sort of heirloom, or as characters belonging originally 
to the typical ancestors. We see thus that these facts not 
only offer no difficulty of explanation by the theory of evolu¬ 
tion, but actually follow as a necessary consequence. The 
very exceptions seem to strengthen our argument. If there 
are such animals as the spiders, which have not twenty 
segments as they ought to have, it is easy to see that 
although the typical character of the progenitors has been 
retained by most of the descendants, in cases of exceptional 
modification even that may have undergone a change ; hut a 
theory of design or necessity is at once made impossible. 
There is more evidence of the same class in the simi¬ 
larity of structure between different organs of the same 
individual. Compare the structure of the human leg and 
arm, of the bird’s leg and wing, of the finger and the toe, 
