70 PERPETUATION OF LIVING BEINGS 



find that the head is like that of the Ass, that the ears are 

 long, the tail is tufted at the end, the feet are small, and 

 tiie voice is an unmistakable bray ; these are all points of 

 similarity to the Ass ; but, on the other hand, the barrel of 

 the body and the cut of the neck are much more like those 

 of the Mare. Then, if you look at the Hinny, the result 

 of the union of the Stallion and the she-Ass, then you 

 find it is the Horse that has the predominance ; that the 

 head is more like that of the Horse, the ears are shorter, 

 the legs coarser, and the type is altogether altered ; while 

 UK' voice, instead of being a bray, is the ordinary neigh of 

 the Horse. Here, you see, is a most curious thing: you 

 take exactly the same elements, Ass and Horse, but you 

 combine 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, necessary 

 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 for 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 tempera- 

 ture, 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 employed : certain it is, that external con- 

 ditions 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 



