153 THE CAUSATION OF DISEASE. 



That the manner in which S responds to E depends in large 

 measure upon the nature of the S is a fact familiar to the 

 biologist. Darwin is very careful to insist upon the important 

 part played by S in determining whether an animal or plant 

 shall vary under a particular E. # He maintains, indeed, that 

 the occurrence of the variation depends more upon the organism 

 (=S) than upon the E. 



It is easy to show that the " nature of the organism" (= S) has 

 much to do with its tendency to variability, for of many organ- 

 isms subjected to practically the same conditions, some will vary, 

 and others will not, and this difference in response to E must 

 depend upon differences in S. Gardeners have often great 

 difficulty in getting plants to vary in a particular direction, 

 and much care is directed towards this end. 



The term ''variation" is generally applied solely to such 

 structural changes as are obvious to the senses, but it 

 should be remembered that the physiological response to a 

 specific E need not display itself in a discoverable structural 

 change. This is sufficiently proved by the fact that many 

 plants do not obviously vary until after they have been 

 exposed to a specific E for several successive generations. 

 There can, however, be no doubt that all this time a steady 

 structural change has been taking place. Indeed, the subtlest 

 and most infinitesimal force falling upon a living organism 

 causes a modification of its structure. Hence, when the E 

 appears to cause a simple functional change only, without any 

 corresponding structural alteration, we may, nevertheless, 

 regard the organism (for the time being, at least) as a natural 

 variation, for there can be little doubt that it does undergo a 

 temporary structural change. I say, a temporary structural 

 change, for, unless the specific E be persistent, the structural 

 change may only be temporary. It need hardly be said that 

 organisms differ quite as much in respect of this "functional 

 reaction," as we may conveniently term it, as they do in 

 regard to the more obvious structural response. Indeed, the 

 latter is but the prolonged result of the former. 



The same remarks apply to pathological variations. In a 



* " Animals and Plants under Domestication," vol. ii. pp. 275-282 (second 

 edit, revised). 



