in PHYSIOLOGICAL VARIATION 119 



different manure more or less alters, between these 

 two extreme data, the proportions of these two im- 

 portant elements of the plant. 



The foregoing instances afford an example of posi- 

 tive measurable variation in chemical constitution ; 

 and if we were better acquainted with the details 

 of the life-history of any species, we should readily 

 perceive the corresponding effects from the physio- 

 logical side : we should see, for instance, how such and 

 such chemical variation, which we can measure and 

 weigh, is of real advantage to those which possess it ; 

 while those without it suffer definite and generally 

 disadvantageous consequences. 



In other cases we perceive the physiological effects, 

 while we are not yet acquainted with the degree or 

 even the nature of the variability. We all know that 

 different animals of the same or of different groups 

 react quite differently under similar unfavourable 

 circumstances. We know, for instance, that it takes a 

 much longer time to drown a frog than a reptile or a 

 bird, and we understand why ; we know also why a 

 duck or penguin can withstand submersion a longer 

 time than a hen or a quail. There are physiological 

 reasons for these facts, and we are familiar with them. 

 But in other cases such reasons must also exist, 

 although we cannot tell what they may be. For 

 instance, if different insects are subjected to the 



