ADAPTATION 191 



of reactions are possible on the same substratum, which 

 increase with regard to one process whilst decreasing at 

 the same time with regard to the other. The irritability 

 of the muscle or of the leaves of Mimosa is a very 

 good instance of the first case, whilst the second more 

 complicated one cannot be illustrated better than by what 

 all experience has taught us about the irritability of the 

 retina. The retina is more irritable by green rays and less 

 by red ones the more it has been stimulated by the latter, 

 and more sensitive to light in general the more it has been 

 exposed to darkness ; and something very similar is true, 

 for instance, as regards phototactic irritability in plants, all 

 these phenomena being in relation to the so-called law of 

 Weber. 1 



It seems to me that there would be little difficulty in 

 harmonising the phenomenon of the inversion of irritability 

 with the so-called principle of the "action of masses' 

 and with the laws of certain " reversible " processes well 

 known in chemistry. As to the simple fact of the re- 



1 I only mention here that certain modern psychologists have assigned 

 the true law of Weber to the sphere of judgment and not of sensation. If 

 applied to objective reactions only, in their dependence on objective stimuli, 

 it, of course, becomes less ambiguous, and may, in a certain sense, be said to 

 measure " acclimatisation " with regard to the stimulus in question. The 

 mathematical analogy of the law of Weber to the most fundamental law of 

 chemical dynamics seems very important. 



As to " acclimatisation " in the more usual meaning of the word, with 

 regard to a change of the general faculty of resisting certain agents of the 

 medium, "immunity" proper is to form a special paragraph of what follows, 

 and to "acclimatisation" towards different degrees of salinity (in algae or 

 fishes) some special remarks will also be devoted on a proper occasion. 

 There remains only " acclimatisation" to different temperatures ; but on this 

 topic not much more than the fact is known (see Davenport, Arch. f. Entw. 

 Mcch. 2, p. 227). " Acclimatisation " does not allow of a sharp general defini- 

 tion : it may be the result of very different kinds of adaptations in our sense 

 of the word. 



