HEREDITARY TRANSMISSION AND VARIATION. 89 



of being a bray, is the ordinary neigh of the Horse. 

 Here, you see, is a most curious tiling : you take ex- 

 actly the same elements, Ass and Horse, but yon com- 

 bine the sexes in a different manner, and the result is 

 modified accordingly. You have in this case, however, 

 a result which is not general and universal — there is 

 usually an important preponderance, but not always 

 on the same side. 



Here, then, is one intelligible, and, perhaps, neces- 

 sary cause of variation : the fact, that there are two 

 sexes sharing in the production of the offspring, and 

 that the share taken by each is different and variable, 

 not only for each combination, but also fur different 

 members of the same family. 



Secondly, 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 which it is ordinarily employed : cer- 

 tain 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 convert single flowers into double 

 flow T ers, 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 



