72 BRITISH LEPIDOPTERA. 



and yellow " determinants," does not help us much, and we un- 

 hesitatingly affirm that whether the scale is finally red, yellow or 

 white, depends primarily on the conditions under which the organism 

 carries on its existence. 



Let us hark back a little. The wing of a butterfly is present in the 

 embryo caterpillar before it hatches from the egg. The wing develops 

 with the caterpillar, and with the pupa, and only ceases to develop 

 with the stretching of the wing following the emergence of the perfect 

 insect. Supposing the animal to have been supplied with a sufficient 

 quantity of suitable nutritious food until the moment of pupation, to 

 have been kept under the most perfect conditions of health throughout 

 its larval and pupal existence, as a result we shall have an imago 

 normal and perfectly typical in size, shape, colour and markings. 

 On the other hand, let the food supply be short and innutritious, or 

 the conditions under which it is reared unhealthy, or let the pupa 

 undergo its final metamorphosis under adverse conditions, and we shall 

 get a specimen small in size, stunted, crippled, maybe failing more 

 or less in colour and modified in markings. Everyone who has bred 

 insects in large numbers knows that these results are certain. These 

 facts are only mentioned to show that these aberrations are outward 

 manifestations of the vital activities of the insect. 



Let us go a little more minutely into the subject. When the 

 pupa of an insect is formed, the tissues (except those connected 

 with the reproductive system) undergo histolysis. They are reduced 

 and changed in character, and, from the material resulting in the de- 

 gradation of the tissues as it were, new tissues are built up by the 

 process of histogenesis. Among others, the scales are formed from epi- 

 thelial cells, and they are filled with a secretion from the ha3molymph, 

 known as "pigment-factor," containing the chemical constituents for 

 pigmentation. The pigmentary matter is deposited on the inside of 

 the scales, the hasmolymph secretion is withdrawn from the scales, and 

 air takes its place. 



The pigment in the scales of insects is of an excretory nature, i.e., 

 it is a product derived from the pnpal blood, and, in a pupa, the larva 

 of which has been reared under typically healthy conditions as to food 

 and environment, this material will be normal ; but if the pupa be 

 weak, due to the unhealthy conditions of food and environment to 

 which the larva has been subjected, will not the material from which 

 the pigment is elaborated suffer with the other tissues, and will not 

 this weakness tend to result in a departure from the normal, i.e., 

 produce an aberration ? 



If it be granted that these conditions are a possible cause of 

 variation, it may readily be surmised that less pronounced changes in 

 the life of a species may produce a less pronounced change in the 

 general appearance, colour, and markings of the individual. It may 

 be that the changes are severe enough to influence, but yet not severe 

 enough to seriously affect, its health. If the change be permanent, 

 then the outward manifestations of the changed vital processes will 

 be exhibited permanently — possibly in the colour and markings. In 

 this manner the differences existing between local races of the same 

 insect living under different environmental conditions may possibly 

 be explained. The predisposing factor to the change may have been 

 food, moisture, heat, cold, or one of many other things, but the factor 



