3i 
1877O Evolution by Expansion. 
an organism, belonging to no matter what class, is capable of 
adaptation to any use, within certain limits, to suit the condi- 
tions of existence of its possessor; moreover, that homologous 
parts have often totally dissimilar functions in different 
species ; for instance, the skull is supposed to be composed 
of metamorphosed vertebrae, and crustaceans having complex 
mouths have few legs, while those with simpler mouths 
have almost invariably more numerous legs ; thus, even 
organs of locomotion may be converted into jaws. There is 
no reason, deducible from the theory of natural selection, 
why, in acquiring a more complex mouth, a crustacean 
should be deprived of some of its legs. It is simply a con- 
sequence of the laws of correlation of growth, and would 
seem to indicate that at a certain period of development the 
different systems of an animal are capable of modification, 
but not of extension. 
This phenomenon of correlative growths is merely another 
form of conformity to type. 
Darwin himself seems to perceive, in this correlation of 
growth, the presence of a higher power than mere natural 
selection. On the subject of domestication he says — 
“ Hairless dogs have imperfect teeth ; long-haired and 
coarse-haired animals are apt, as is asserted, to have long 
or many horns ; pigeons with feathered feet have skin be- 
tween their outer toes, pigeons with short beaks have small 
feet, and those with long beaks large feet. Hence if a man 
goes on selecting, and thus augmenting any peculiarity, he 
will certainly modify unintentionally other parts of the 
structure, owing to the mysterious laws of correlation .” 
What is this mystery but the existence of an innate 
force ? What are these laws of correlation but the evidences 
of its existence ? 
The existence of rudimentary parts is essentially a con- 
formation to type, and another phase of correlation of 
growth. It perhaps shows, more clearly than any facts yet 
mentioned, the nature of the principle of expansion. Rudi- 
mentary parts should be regarded not as organs or members 
in a state of transition, but as developments correlative to 
those of certain other portions of the organism that are in 
use ; for it can abundantly be shown that rudiments of all 
kinds exist in almost every organic strudture that not only 
are useless to the individual, but that never can have been, nor 
ever can be, of any use to the race. The only way of explaining 
the fact of their existence is by the supposition that organisms 
do not change their characteristics one by one, but all simul- 
taneously by a slow process of growth, so that no one part 
