SENSITIVENESS. 71 



under one set of circumstances, of the influence of one 

 agency (controlled or not by others), at another time of a 

 different agency. This affords an explanation of the fact 

 that the seasons marked by extraordinary productiveness are 

 not those wherein some one or more of the conditions have 

 been specially favourable at a particular time, even though 

 that time be the growing period, but those in which the 

 conditions have been generally propitious throughout. 

 The physiologist endeavours to isolate the agencies which 

 influence growth in order to ascertain precisely what each 

 does independently of the others; the practical man has to 

 deal with the combined effect of all, but it is clear that 

 the combination cannot be properly understood unless the 

 separate effect of each component be first clearly compre- 

 hended. 



