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THE EVOLUTION THEOEY 



the germ-plasm is composed of determinants, depends. There must be 

 as many of these as there are regions in the fully-formed organism 

 capable of independent and transmissible variation, including all the 

 stages of development. Every part, for instance, of the butterfly's 

 wing, which is capable of independent and transmissible variation, 

 must, so I conclude, be represented in the germ-plasm by an element 

 which is likewise variable, the determinant ; but the same must be 

 true of every independently and transmissibly variable spot of the 

 caterpillar from which the butterfly developed. We know how 



Fig. 17. Caterpillar of Selenia tetralunaria on a twig of bircli. K, head. 

 F, feet, m, protuberances resembling dormant buds. Natural size. 



markedly caterpillars are adapted in form and colour to their environ- 

 ment. Let us assume that the caterpillar of the butterfly which we 

 chose as an example of wing-marking had the habit of feeding only 

 by night and during the daytime of resting on the trunk of a tree, or, 

 more precisely, in the crevices of the bark. It would then resemble 

 the caterpillar of the moths of the genus Catocala or the Geometers 

 (Geometridse), and possess the colour of the bark of the tree in 

 question; the determinants of the skin would thus have varied to 

 correspond with this mode of life on the part of the caterpillar, so that 

 the skin would appear grey or brown. But there cannot be only one 



