4,02 THE CAUSES OF THE <r 
but also for different members of the same 
family. 
Secondly, there. is a variation, to a certain ex- 
tent—though, in all probability, the influence of 
this cause has been very much exaggerated—but 
there is no doubt that variation is produced, to a 
certain extent, by what are commonly known as 
external conditions,—such as temperature, food, 
warmth, and moisture. In the long run, every 
variation depends, in some sense, upon external 
conditions, seemg that everything has a cause of 
its own. I use the term “ external conditions” 
now in the sense in which it is ordinarily em- 
ployed : certain it is, that external conditions have 
a definite effect. You may take a plant which has 
single flowers, and by dealing with the soil, and 
nourishment, and so on, you may by and by con- 
vert single flowers into double flowers, and make 
thorns shoot out into branches. You may thicken — 
or make various modifications in the shape of the 
fruit. Inanimals, too, you may produce analogous 
changes in this way, as in the case of that deep 
bronze colour which persons rarely lose after 
having passed any length of time in tropical coun- 
tries. You may also alter the development of the — 
muscles very much, by dint of training; all the 
world knows that exercise has a great effect in this - 
way ; we always expect to find the arm of a black- 
smith hard and wiry, and possessing a large 
development of the brachial muscles. No doubt 
