222 Transactions British Mycological Society. 



those investigators who have hghtly paid allegiance to the 

 general concept: and brief attention may profitably be given 

 to certain of these issues. 



The great mass of evidence which we have at our disposal 

 would appear to point to the fact that the essential physiological 

 nature of an organism is a constant. Now the physiological 

 constitution of an organism interacting with certain physico- 

 chemical factors in a particular environment produces a 

 certain resultant — the visible individual — the characteristic 

 properties of which are described in terms of colour, size, 

 shape, etc. If the physiological constitution and the environ- 

 mental factors are both constants it follows that the resultant of 

 their interaction must also be a constant however often it may 

 recur. But if apparently identical physiological constitutions 

 under apparently identical conditions produce two different 

 resultants it must follow either 



(i) That the conditions are not really identical but different, 

 for otherwise things which are equal to the same thing would 

 not be equal to one another. 



Or (2) that for a similar reason the physiological constitu- 

 tions are different, i.e., that some change in the ultimate 

 physiological nature of the organism must have occurred. 



A graphic expression may be given to this as follows : 



Let a = the physiological constitution of an organism, 

 and let h = the constant environmental conditions ; 

 then ab = the resultant of interaction which is the visible 

 individual. 



Now if a X 6 on one occasion produce " ab," and on another 

 occasion produce some other resultant such as " x," it must 

 follow that either or both <x and b have changed from their 

 original state into some different constants such as "c" or "^." 



If this change be one in the environmental conditions the 

 resultant is merely a "modification " and outside this discussion. 

 If, however, the change be in the physiological constitution of 

 the organism it is a fact of incalculable import, for it would 

 mean that the physiological constitution of an organism could 

 be altered at will, the facility and scope of change increasing 

 with fuller knowledge and refined technique so that theoretically 

 there is no limit to its operation. But the only foundation 

 which we can find for any systematic categorisation of living 

 organisms is in the absolute constancy of their physiological 

 constitution, this being the ultimate specific criterion. If 

 therefore one may at will transmute one physiological constitu- 

 tion into another, it follows that one can change one species 

 into another species, and this with great rapidity, merely by the 

 simplest adjustment of the most common environmental factors. 



