150 ACQUIRED CHARACTERS sec. 



Kocli employs the facts he brings forward to support a 

 theory set up by him concerning tlie origin of the colours, 

 which physiologically is somewhat audacious. According to 

 this theory, a yellowisli slime overlies the imago's wings before 

 its emergence. This slime consists of granules, which after- 

 wards form the pigment particles of the imago. The cater- 

 pillars obtain the substance of this slime from the plants they 

 feed on in the form of vegetable salts, acids, and tannin. Koch 

 was, I believe, originally a simple artisan of Frankfurt (a tin- 

 man), and we cannot be severe upon him for explaining the 

 production of colour as a simple laying on of pigment. On 

 the other hand, it is highly gratifying to see facts employed 

 by such a man scientifically, that is, so as to afford general 

 conclusions, so that it must be said of him, that with simple 

 unskilled judgment he hits the nail on the head. 



Koch guards himself against the charge of not recognising 

 the action of light on the formation of colours : he says this 

 action is shown by the fact that diurnal Lepidoptera all have 

 more vivid colours than the nocturnal. Warmth and light, 

 however, according to him, do not act differently. 



Koch's results are as follows : — 



A change in marking is easily produced by a change in the 

 plants fed upon (the "bear" and other species). 



Nocturnal species which live exclusively on conifers have 

 dull colours, usually gray, as for example our pine hawk- 

 moth (Sphinx pinastri), or the pine -spinner (Gastropacha 

 pini), and several foreign species. This is so invariably true 

 that Koch was able to conclude from the colours of certain 

 species from Sydney and Baltimore that the caterpillars lived 

 upon coniferous plants, and when he suggested that they should 

 be sought on such plants his conclusion was found to be correct. 



It is known, Koch says, that when the caterpillars of our 

 German " bears " (Chelonia s. Euprepia caja) are fed from 

 their hatching to their metamorphosis with leaves of Lactuca 



