HEREDITARY TRANSMISSION AND VARIATION. 93 



Secondlyj there is a variation^ to a certain extent, — 

 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 ex- 

 ternal conditions, seeing that everything has a cause 

 of its own, I use the term "external conditions " now 

 in the sense in Avhich it is ordinarily employed : certain 

 it is, that extei-nal 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 convert 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 countries. 

 You may also alter the development of the muscles 

 very much, by dint of training ; all the world knows 

 that exercise has a great eff'ect in this way ; we always 

 expect to find the arm of a blacksmith hard and wiry, 

 and possessing a large development of the brachial 

 muscles. No doubt, training, which is one of the forms 

 of external conditions, converts what are originally 

 only instructions, teachings, into habits, or, in other 

 words, into organizations, to a great extent ; but this 

 second cause of variation cannot be considered to be 

 by any means a large one. The third cause that I 



