THE GEEM-PLASM THEORY 361 



determinant of the caterpillar skin in the germ-plasm, for the bark- 

 like colour of, for instance, a Geometer caterpillar is not a uniform 

 grey, but has darker spots at certain places and lighter whitish spots 

 at others, such as are to be seen on the bark of the twig on which 

 the caterpillar is wont to rest, or brown-red spots, like those on the 

 cover-scales of the buds, or little warts and protuberances which exactly 

 correspond to similar roughnesses on the twigs, to cracks in the bark, 

 and so on. All these markings are constant, and are to be found 

 in the same spot in every caterpillar of the species. A- large number 

 of regions of the caterpillar skin must therefore be independently 

 determined by the germ-plasm ; the germ-plasm must contain parts 

 the variations of which bring about variations only of an independently 

 variable region of the caterpillar skin. In other words, in the germ- 

 plasm of the butterfly ovum there must not only be determinants for 

 manj^ regions of the butterfly's wing, but also for many regions of the 

 caterpillar's skin. 



This line of argument, of course, applies to all the bodily parts 

 and organs of the butterfly and of the caterpillar, as well as to 

 all the stages of development of the species as far as these parts 

 are able to vary in such a way that the variation reappears in the 

 following generation, that is to say, as far as it is transmissibly 

 variable. 



But all parts must be transmissibly variable which have ex- 

 hibited independent variation in relation to their ancestors. When, 

 for instance, the eggs of a butterfly (Vanessa levana) bear a deceptive 

 resemblance to the flower-buds of the stinging-nettle on which the 

 caterpillar lives, not only in form and colour, but in their pillar-like 

 arrangement, we may conclude that these eggs have varied trans- 

 missibly from those of their ancestors, which had not acquired the 

 habit of living on the stinging-nettle, in these three respects inde- 

 pendently, that is, uninfluenced by any other variations the species 

 may have undergone ; and that, consequently, the germ-plasm must 

 contain determinants for the egg-shell, egg-colouring, and so on. 

 The manner of laying the eggs in the form of pillars depends on a 

 modification of the egg-laying instinct, which must in its turn depend 

 on the variations of certain nerve-centres, and we learn from this 

 that there must be in the germ-plasm determinants for the indi- 

 vidual centres of the nervous system. 



It may, perhaps, be suggested that matters could be explained 

 in a simpler way— that it is enough to assume the presence in 

 the egg of determinants for all the parts of the caterpillar, and that 

 those of the butterfly are only formed within the caterpillar. 



