CH. XIX Influence of Habits and Surroundings 315 



glacial epoch there was perhaps only A. levana, the winter 

 form ; the change of climate has perhaps evolved the 

 summer variety A. prorsa. Both Weismann and Edwards 

 have succeeded, by artificial cold, in making the pupas which 

 should become the summer A.firorsa develop into the winter 

 A. levana. Nor can we forget the seasonal moulting and 

 the subsequent change of the plumage in birds, so marked 

 in the case of the ptarmigan, which moults three times in 

 the year. In the puffins even the bill is moulted and 

 appears very different at different seasons. But in these 

 last cases the ' influence of environment must be very 

 indirect. 



Light is very healthful, but it is not easy to explain its 

 precise influence. Our pulses beat faster when we go out 

 into the sunlight. Plants live in part on the radiant 

 energy of the sun, and perhaps some pigmented animals do 

 the same. Perhaps the hundreds of eyes which some mol- 

 luscs have are also useful in absorbing the light. It is also 

 possible that light has a direct influence on the formation o< 

 some animal pigments, as it seems to have in the develop- 

 ment of chlorophyll. We know, from Poulton's experiments, 

 that the light reflected from coloured bodies influences the 

 colouring of caterpillars and pupse, but this influence seems 

 to be subtle and indirect, operating through the nervous 

 system. It is also certain that hving in darkness tends to 

 bleach some animals, and it is probable that the absence 

 of light stimulus has a directly injurious effect upon the 

 eyes of those animals which live in caves or other dark 

 places. But I have already explained why dogmatism in 

 regard to these cases should be avoided. 



One case of the influence of light seems very instructive. 

 It is well known that flat fishes like flounders, plaice, and 

 soles lie or swim in adult life on one side. This lower side 

 is unpigmented ; the upper side bears black and yellow 

 pigment-containing cells. 



One theory of the presence of pigment on the upper 

 side and its absence on the other is that the difference is 

 a protective adaptation evolved by the natural selection of 

 indefinite variations. But it is open to question whether the 



