402 THE CAUSES OF THE X l 



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, seeing 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. In animals, 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 



