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the results cf particular forms of physiological activity (or 

 inactivity). 



There can be no doubt that the development of pigment 

 in animals generally is to be associated with the physio- 

 logical functions of the animal, and anything that would 

 stimulate or weaken these functions would, in the case of 

 insects, have its reaction on the colour produced in the final 

 stage of the creature. But with regard to insects there is 

 some difficulty, for the food material, so far as it goes 

 towards and therefore is capable of influencing the formation 

 of pigment, is simply stored up in the larval stage and con- 

 verted into pigment in the pupal stage. We know that 

 ill-fed larvse of many species produce diminutive and ill- 

 pigmented imagines ; but the direct influence of food, 

 except in its effect on the size of the resulting imago, is not 

 easily demonstrable, although large and full-sized imagines 

 are usually well pigmented. In the same way the effect of 

 temperature, &c., on larvse is not easily demonstrable, but 

 the effect of any external factor — cold, excessive heat, &c. — 

 that will affect the physiological functions of the pupa at 

 the time that the pigment factor is being elaborated in the 

 scales is almost certain to be marked in the imago. 

 Merrifield's experiments prove this up to the hilt, and there 

 is no doubt that any modification of the normal activities at 

 this time will result in a difference of colour. Not, of 

 course, in all species, for the variability in this direction is 

 not elastic in all species, and the external influence that 

 would simply produce a change (more rapid or less rapid) 

 in the functions of one insect might arrest the vital functions 

 of another, and produce death. And even in the case of a 

 species with a certain degree of variability in the required 

 direction there would certainly be some that would 

 not respond, and others that would die under the treat- 

 ment that would only create a change in others, 

 and hence there would be a destruction of the physio- 

 logically unfit ; and it is clear that what at first seems 

 to be the transformation of a species by the influence of 

 external temperature or climatic conditions is in reality a 

 selection of those individuals that respond best to the 

 changed conditions, the effect on those individuals that can 

 undergo a more rapid or less rapid metabolism, as the case 

 may be, being an external change of colour, which is in 

 reality correlated with a somewhat different physiological 

 organisation. But the selection of individuals which will 

 respond to these changed conditions of environment, and the 



