CORRELATED VARIATIONS. 95 



ductive system slightly altered, whereby the principles 

 of Genetic Selection and in the case of plants of 

 Physiological Selection, may become effective, and 

 modify or split up the species. 



Just as the condition of the organism as a whole may 

 modify that of the reproductive system, so may the con- 

 dition of the reproductive system modify that of the 

 organism. The difference between the spirit and ap- 

 pearance of castrated animals and th#t of normal ani- 

 mals is sufficiently well known to need no remark. 

 Such differences must be due in large part to the lack of 

 internal secretion from the organs of reproduction. 

 Striking as is the influence of the reproductive organs 

 on the physiological condition of an animal, that upon 

 the morphological structure is even more noteworthy. 

 That castrated male animals fail to develop their sec- 

 ondary sexual characteristics, is notorious. Thus, if the 

 operation be performed upon a young cock, he never 

 crows again; the comb, wattles, and spurs do not grow 

 to their full size, and the hackles assume an inter- 

 mediate appearance between true hackles and the 

 feathers of the hen. Conversely, it is well known that 

 a large number of female birds, such as fowls, various 

 pheasants, partridges, pea-hens, and ducks, when old or 

 diseased, or when operated upon, assume many or all of 

 the secondary male characters of their species. Water- 

 ton gives a curious case of a hen which had ceased lay- 

 ing, and had assumed the plumage, voice, spurs, and 

 warlike disposition of the cock. Again, the females of 

 two kinds of deer, when old, have been known to ac- 

 quire horns.* 



"Animals and Plants," vol. ii. p. 26. 



