50 PROBLEMS OF EVOLUTION 



If the frog happens to be blind, no change of colour takes place, 

 so that it is by the help of the eye and the nervous system that 

 the change is effected. In Mr Poulton's book on the Colours of 

 Animals will be found an account of the strange power possessed 

 by the pupae of some butterflies of adapting their colour to the 

 surroundings among which they are placed. Here too the 

 marvel lies in the wonderful power of response to environment 

 possessed by the pupa. It would be strange beyond all com- 

 prehension if the environment were able to endow the pupa 

 with its own hue. Animals may live their whole life long in 

 evergreen forests and yet their skin, hair or feathers may show 

 not a speck or a tinge of green. They have no susceptibility to 

 this particular kind of stimulus. When the American hare turns 

 white in winter the old hairs lose their colour, the change usually 

 beginning at the tip, and a great many new completely white 

 hairs appear. The animal, in fact, has the power of turning 

 white when the cold season comes on and of producing a fresh 

 crop of white hair to keep itself warm. It is difficult to see how 

 the cold could cause hair to grow or to change colour, unless the 

 animal in question had the power of reacting to this particular 

 stimulus. In fact an external condition can do nothing but bring 

 to light some latent quality. Speaking casually and unscientifi- 

 cally, we may say that being a member of the House of Commons 

 has made so-and-so a fine debater. To put it more correctly, 

 it has developed a power that was already there. Weismann 

 expresses this tersely and well: "Nothing can arise in an 

 organism unless the pre-disposition to it is pre-existent, for 

 every acquired character is simply the reaction of the organism 

 upon a certain stimulus." 1 



A few more instances will help .to illustrate this. Ononis or 

 Restharrow varies a great deal in character according to the 

 situation in which it grows. On maritime sands it is armed 

 with strong thorns. 2 When growing in rich soil, and in a 

 moist climate, it is often entirely devoid of them. It has in 



1 Essays, vol. i. p. 172. 



2 When growing by the sea-side and not in a dry sandy soil it is often quite free 

 from thorns. On plants in relation to their environment, see Henslow, Journal of 

 Linn<ean Soc. (Botany), vol. xxx. p. 218 (1895). There is much valuable in- 

 formation there though I cannot accept his theory. 



