The Evidence from Hybrids. 133 



variation. As the gem mule and the ovarian element are 

 both very similar to those which produced the variation 

 in the ancestor, the chances are -not very great against 

 the reproduction of the same peculiarity. In this case 

 we should have a new variation with all the characteris- 

 tics of a true reversion, but due to the transmission of a 

 gcmmule, rather than to the sudden awakening of a 

 tendency which has long lain dormant in the egg. 



It is possible, therefore, that there may be two kinds 

 of reversion true hereditary reappearance of features 

 which have lain latent in the egg, and new variations 

 which repeat again certain old characteristics of the race. 

 There are, I think, certain reasons for believing that re- 

 versions of the latter kind are the most common, the 

 chief one being the fact that most of the causes of vari- 

 ability arc also causes of reversion. 



Thus, crossing, which is a very efficient cause of varia- 

 tion, is also one of the chief causes of reversion. 



Darwin giyes a number of examples to show that, in- 

 dependently of the well-known .tendency of hybrids and 

 mongrels to revert, after a number of generations, to one 

 of the parent forms, the act of crossing in itself gives an 

 impulse towards reversion, and often results in the reap- 

 pearance of long-lost characters. 



The following interesting account, from Darwin's Va- 

 riation (Vol. ii. p. 57), will serve to illustrate this law: 



" In the chapter on the horse, reasons were assigned 

 for believing that the primitive stock was striped and 

 dun colored, and details were given showing that in all 

 parts of the world stripes of a dark color frequently ap- 

 pear along the spine, across the legs and on the shoul- 

 ders, where they are occasionally double or treble, and 

 even sometimes on the face and body of horses of all 

 breeds and of all colors. But the stripes appear most 



