48 
of altering the focus of the lenses so as to suit divers distances. 
This power is, however, not only naturally variable, but so 
little constant as to be considerably altered by use m a parti- 
cular direction, even through a comparatively short space o 
time. It can be increased in either direction ; but this increase 
brings with it a corresponding’ limitation in the other. Similar 
alterations in structure by use in respect to the ear mi g t 
easilv be instanced. So, Again, there is no doubt that the size 
and proportional development of the stomach ana other internal 
organs are directly influenced by the nature of the food habitu- 
ally eaten ; the size of the lungs by the temperature of the air 
breathed, and the occupation of their possessor. 
These are the kind of elements with which natural selection 
has to deal : differences in structure, physiological character, 
instinct, and habit ; differences, some of them directly occa- 
sioned by the conditions of life, the use and disuse of particular 
organs, some of them by causes existent before biith, and o 
which we know nothing; differences all certainly transmissible 
by inheritance. To appreciate the wide extent of the ground 
covered by such known individual variations vould require a 
detailed survey of facts infinitely luller than the bare outline 
here afforded, which, as already remarked, merely professes to 
oflve illustrations of the different hinds of variations observ- 
able. We may, however, even from this meagre view, un- 
hesitatingly conclude thus much : — that there is no class of 
specific differences which facts do not. fully warrant us in 
regarding as possibly caused by inherited variations, lhe 
amount of such differences will come under consideration 
further on. 
But now, in the next place, of the power and extent of 
natural selection as a process for preserving and confirming 
such variations. Here we need carefully to bear in mind the 
exceedingly complex relations in which all living beings stand. 
There are first their relations to inanimate nature, to soil, 
climate, and situation. Then there are. their direct relations 
to one another, the presence of one being necessary to the 
well-being or existence of another, or acting as a check upon 
its development ; so that the increase or decrease of one will 
entail at once the increase or decrease of the other. Then, 
thirdly, there are their indirect relations, caused by that compe- 
tition of races and individuals before dwelt upon ; those, which 
do not directly affect each other' s well-being, yet struggling to- 
gether for existence, by reason of a greater number of germs 
being constantly produced than can possibly attain to matuiit) . 
These various relations affect species in every part of their 
