IN THE THEORY OF NATURAL SELECTION. 303 



protecting reaction under a stimulus, and that therefore an organism 

 does not always respond to external stimuli in a manner useful to 

 itself. 



But even if we could accept the suggestion that the purposeful 

 reaction of an organism under stimulation is a primary and not a 

 secondarily produced character, such a principle would by no means 

 suffice for the explanation of existing adaptations. Nageli attempts 

 to explain certain selected cases of adaptation as the direct results 

 of external stimuli. He looks upon the thick hairy coat of mam- 

 mals in arctic regions, and the winter covering of animals in tem- 

 perate regions, as a direct reaction of the skin under the influence 

 of cold. He considers that the horns, claws, and tusks of animals 

 have arisen directly as reactions under stimuli applied to certain 

 parts of the surface of the body in attack and defence *. This inter- 

 pretation is similar to that offered by Lamarck at the beginning of 

 this century. At first sight such a suggestion appears to be 

 plausible, for the acquisition of a thick hairy covering by the 

 mammals of temperate regions is actually contemporaneous with 

 the cold season of the year. But the question arises as to whether 

 the production of a larger number of hairs at the beginning of 

 winter is not merely another instance of a secondary character, like 

 the assumption of a green colour by the tree-frog under the stimulus 

 exerted by strong light. 



In the case of the hairy coat it is only necessary to produce a 

 larger number of structures such as had existed previously ; but how 

 can- it have been possible for the petals of flowers, with their 

 peculiar and complex forms, to have been developed from stamens 

 as a direct result of the insects which visit them in order to obtain 

 pollen and nectar ? How could the creeping of these insects and 

 the small punctures made by them constitute stimuli for the produc- 

 tion of an increased rate of growth ? And how is it possible in any 

 way to explain, by mere increase in growth, the origin of a struc- 

 ture in which each part has its own distinct meaning and plays 

 a peculiar part in attracting insects and in the process of cross- 

 fertilization effected by them ? Even if the manifold peculiarities of 

 form could be explained in this way, how can such an explanation 

 possibly hold for the colours of flowers? How could the white 

 colour of flowers which open at night be explained as the direct 



1 1. c., p. 144. 



