CROSSING AND CLOSE-INTERBREEDING. 235 



parents having something to bestow, where existed de- 

 fects in the coordination of the other. 



If proof of this principle be required, observe all of 

 the cases of crossing; note the amount of structure, 

 which is possessed by either parent, and which is 

 wanting in the other; mark the advance towards 

 structural integrity, made upon the parents, by the off- 

 spring; and, then observe the constant relation, sub- 

 sisting between the gain, to the offspring, 'in fertility, 

 and in constitutional vigor, and the advance, made 

 by such offspring, in mere, structural development. 



Note the great improvement, in fertility, and in con- 

 stitutional vigor, resulting from crossing two widely 

 divergent varieties, of. pigeons, which have, each, an 

 important; and strongly pronounced character, which 

 the other does not possess. Note, on the other hand, 

 the comparatively little increase in fertility, and in 

 vigor, which follows from crossing two well-bred 

 varieties of the horse, of the sheep, or of the cow 

 species, which are distinguished from each other, by 

 but some slight differences in the ratio of the develop- 

 ment of their characters. 



In order to demonstrate, that the good effects, occa- 

 sioned by crossing, are due to the increased return, 

 made by the offspring, to the structure of the original 

 type, which possessed all of the characters of the 

 given species, it is not necessary to invent any meta- 

 physical entity; to seek refuge behind any "great law 

 of nature," fashioned for the nonce ; nor to appeal to 

 any gratuitous supposition, which is, besides, incompe- 

 tent to explain the many differences in the quantity of 



