94 CORRELATED VARIATIONS. 



the reproductive system is closely correlated with the 

 physiological condition of the organism taken as a 

 whole. Recent physiological research has taught us 

 that most organs in the body, in addition to their more 

 obvious functions, have an internal secretion which 

 passes into the circulation of the body, and there exerts 

 some important, but unexplained, influence on the gen- 

 eral metabolism of the tissues. Deprivation of such 

 internal secretion, by extirpation of one of these organs, 

 may speedily upset the normal working of many or 

 most of the other tissues of the body, and ultimately re- 

 sult in death. Every organ and tissue of the body 

 probably reacts on every other organ, and modifies its 

 physiological condition, and thereby may ultimately 

 produce structural changes. The reproductive system 

 is apparently more sensitive than most other organs, 

 and hence is very readily affected by any changes in the 

 condition of the organism as a whole. For instance, it 

 is well known that most animals refuse to breed in con- 

 finement, though they can be kept for many years in a 

 condition of perfect health. The changed conditions 

 of life must therefore have acted on the organism as a 

 whole, so as to modify certain of its internal secretions, 

 and these, reacting on the reproductive system, have 

 brought about the observed sterility. If a considerable 

 change in conditions of life produces complete sterility, 

 it seems highly probable that slight changes in such 

 conditions may produce a partial sterility, or a differ- 

 ential fertility. Thus organisms in a state of nature, 

 if exposed to a change of climate, the result of migra- 

 tion or stress of weather, or to a change in their food, 

 may have the physiological condition of their repro- 



