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Evolution by Expansion. 
I have seen it stated that the theory of natural selection 
does not suppose the individual to be capable of self- 
improvement, but that purely fortuitous variations are ren- 
dered permanent by inheritance provided they happen to be 
improvements, and that thus only have the different forms 
of life been produced. But Darwin certainly believes that 
some variations are due to effort, or use and disuse, caused, 
however, entirely by the accidental circumstances of the 
conditions of life, and that variations that are inherited 
become characteristics of new species.* Although he has 
not expressly excluded the possibility of the aCtion of some 
other agency, the general tenour of his writings is sufficient 
to convince us that in his opinion natural selection is suffi- 
cient to account for the origin of species. The only instances 
in which he appears to entertain misgivings on this point 
have been already mentioned. 
The chief difficulty of the theory of expansion is the faCt 
of the existence at the present time of the lowest forms of 
life. Several explanations suggest themselves - 
First. The germs of life may exist inactive for many 
ages, until activity is set up by some particular cause* 
Secondly. The germs of life may be capable of sponta- 
neous generation. 
Thirdly. Certain germs may have a limited power of 
development. 
Fourthly. Certain germs may be retarded or completely 
arrested in their development from some cause or other. 
These suggestions are purely speculative ; they are given 
merely to show that the difficulty admits of explanation in 
various ways. 
*1 have chosen the word “ expansion ” as the one open to the 
fewest objections when used in its mathematical sense. The 
word “ growth ” would confuse the age-development with the 
growth towards maturity of the individual, from which it is 
quite distinct. The words “ development ” and “ evolution ” 
express only improvement or extension, but “ expansion ” 
conveys the idea of development by the aCtion of some force 
or agent, or else extension according to a definite law, such 
as the expansion of a binomial into an infinite series. t 
* See Preface to second edition of “ Descent of Man.” 
f The word “ Expansion ” has been chosen, also, to distinguish the theory 
from that of “ Progressive Development ” advanced by Lamarck, and which 
he admits to be merely a statement of observed fads, and from those that 
suppose the existence of an innate tendency to improvement and perfection ; 
in short, to distinguish a process of “unfolding” from one of “ addition.” 
