APPLICATION TO METAZOA. 131 



The effect of changes in the body upon the germ- 

 cells is a subject at which we can only arrive by 

 indirect evidence; but when we come to examine 

 the converse of the proposition, the effects of the 

 germ-cells upon the body, shown by their pres- 

 ence or absence, a mass of direct evidence lies 

 at hand. It is a matter of almost universal obser- 

 vation. The removal of the germ-cells seems to 

 affect profoundly the constitution of the animal. 

 If the operation be performed on a cow at a period 

 of full flow of milk, the secretion of milk is said, 

 in some cases, to become permanent. If performed 

 on a very young cow, the growth becomes more 

 rapid and the beef will be more marketable. It 

 might be supposed from this that the germ-cells 

 act in some way as secretory glands, diverting 

 nutriment from the body ; but that this is not their 

 only nor their strongest action is shown by the 

 marked change of instinct caused by their degen- 

 eration or eradication. In his book, Animals and 

 Plants under Domestication, Darwin speaks as fol- 

 lows on this subject: — 



"It is well known that a large number of female 

 birds, such as fowls, various pheasants, partridges, 

 peahens, ducks, etc., when old or diseased, or 

 when operated on, assume many or all of the sec- 

 ondary male characters of their species. In the 



