Chap. XXL 
AND CONCLUDING EEMARKS. 
387 
consequently he is occasionally subjected to a severe 
struggle for existence, and natural selection will have 
effected whatever lies within its scope. A succession 
of strongly-marked variations of a similar nature are 
by no means requisite ; slight fluctuating differences 
in the individual suffice for the work of natural selec- 
tion. We may feel assured that the inherited effects 
of the long-continued use or disuse of parts will have 
done much in the same direction with natural selection. 
Modifications formerly of importance, though no longer 
of any special use, will be long inherited. When one 
part is modified, other parts will change through the 
principle of correlation, of which we have instances in 
many curious cases of correlated monstrosities. Some- 
thing may be attributed to the direct and definite 
action of the surrounding conditions of life, such as 
abundant food, heat, or moisture ; and lastly, many 
characters of slight physiological importance, some in- 
deed of considerable importance, have been gained 
through sexual selection. 
No doubt man, as well as every other animal, pre-» 
sents structures, which as far as we can judge with 
our little knowledge, are not now of any service to 
him, nor have been so during any former period of his 
existence, either in relation to his general conditions of 
life, or of one sex to the other. Such structures cannot 
be accounted for by any form of selection, or by the 
inherited effects of the use and disuse of parts. We 
know, however, that many strange and strongly-marked 
peculiarities of structure occasionally appear in our 
domesticated productions, and if the unknown causes 
which produce them were to act more uniformly, they 
would probably become common to all the individuals 
of the species. We may hope hereafter to understand 
something about the causes of such occasional modi- 
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