ZOOLOGY. 241 



and have enforced it in tlieir writings on tlie subject of organic selection. 

 The contention here urged is that natural selection worlvs upon the highest 

 organisms in such a way that they have become modifiable, and that this 

 poAver of purely individual adaptability in fact acts as the nurse by whose 

 help tlie .species, as the al)ove named authorities maintain, can live through 

 times in which the needed inherent variations are not forthcoming, but 

 in part acts also as a substitute, not indeed for natural selection, but for 

 the ordinary operation by which the latter produces change. In this latter 

 case natural selection acts so as to produce a plastic adaptable individual 

 which can meet any of the various forces to which it is likely to be ex- 

 posed by producing the appropriate modification and this, it is claimed, 

 is in many instances more valuable than the more perfect, but more rigid, 

 adjustment of inherent variations to a fixed set of conditions. 



A good example of the eminent advantages of adaptability in many 

 directions, over accurate adjustment in fewer directions, is to be found 

 in a comparison between the higher parts of the nervous system in insects 

 and birds. The insect performs its various actions instinctively and per- 

 fectly from the first; it is almDSt incapable of education and of modify- 

 ing its actions as the result of the observation of the effects of some new 

 danger. It would appear that the introduction of the electric light can 

 only afiect the insects which are most attracted to it, by the gradual oper- 

 ation of natural selection. In the clothes-moths, which infest our houses, 

 we may see aji example of this ; for these insects seem to be compara- 

 tively indifferent to light. Birds, on the other hand, have the power of 

 learning from experience, of reasoning from the results of observation. 

 At first terrified by railway trains they learn that they are not dangerous, 

 and cease to be alarmed ; while the efl'ect of firearms results in their in- 

 creased wariness. 



If this view of individual adaptability as due to natural selection be 

 not accepted it may be supposed that the individual modifications are due 

 either to the direct action of the external forces, or to the tendencies of 

 the organism. But it is impossible to understand how the mechanical oper- 

 ation of such forces as pressure, friction, stress, etc., continued through 

 a lifetime, could evoke useful responses, or why the response should just 

 attain and then be arrested at a level of maximum efl^ciency. The other 

 supposition, that organisms are so constituted that they mws^ react under 

 external stimuli by the production of new, useful characters, or the use- 

 ful modification of old ones, seems to me to be essentially the same as the 

 old " innate tendency toward perfection" as the motive cause of evolu- 

 tion — a conception which is not much more satisfactory than special crea- 

 tion itself. The inadequacy of the view is clearly shown when we consider 

 that the external forces which awake response in an organism generally 

 belong to its inorganic (physical or chemical) environment, while the use- 

 fulness of the response has relation to its organic environment (enemies, 

 prey, etc.). Thus one set of forces supply the stimuli which evoke a re- 

 sponse to another and very different set of forces. We can therefore 



