VARIATION 425 



' factor ', that the effects of a { factor ' may be far-reaching 

 and manifold, and that a single character may depend on 

 many i factors ' which interact. " Cases of interaction of 

 factors, in which the effect of one factor is altered by the 

 action of another factor, are very numerous " (p. 46). " The 

 expression of a factor-difference may not be limited to one 

 region but may produce a different effect in different re- 

 gions." 



Many considerations suggest that we should do well to ap- 

 preciate afresh the idea which Darwin and Sir Ray Lan- 

 kester have emphasised of the " correlation of variations ", 

 that one change, as we see for instance in disease, may have 

 manifold expression or outcrop in different parts of the 

 body, that the organism may change as a unity in many parts 

 at once. It is not difficult to suppose that a change in the 

 rate of a particular kind of metabolism may reverberate 

 through the body. As Mr. J. T. Cunningham and Professor 

 Dendy have pointed out, an augmentation or a diminution 

 of certain internal secretions or hormones might have 

 multitudinous transforming effects. 



6. Theory of Temporal Variations. 



Another important idea is that of temporal variations, 

 that is to say alterations in the tempo, or rate, or rhythm 

 of metabolic processes, or in the duration of particular 

 phases in the life-cycle. Many changes of great adaptive- 

 ness are probably due to a lengthening out of one chapter 

 and the telescoping of another. In the remarkable regula- 

 tory influence of the internal secretions in backboned an- 

 imals we get a hint as to the way in which changes in l time ' 

 might be effected. 



It is very interesting to compare different life-histories 



