ASPECTS OF KINETIC EVOLUTION 257 



tive response to the special demands. In groups subjected to 

 an active struggle for existence this would mean a change of 

 direction rather than a cessation of selection. In many other 

 instances, notably among parasitic forms, the loss of normal 

 organs ascribed to disuse is better explainable by selection, since 

 the apparent degeneration is of decided advantage from the 

 standpoint of the actual life-history of the animals. 



The principle of panmixia seems, indeed, to involve an un- 

 warrantable extension of the idea of organic elasticity, since it 

 implies that organic structure is maintained by selection alone, 

 without which everything would drop back to simple protoplasm. 

 Of such a general tendency to degeneration there is, however, 

 no indication. As explained elsewhere, the reversion of inbred 

 highly selected types to the wild form of the species is not de- 

 generation, but a recovery of normal structure after restoration 

 to normal conditions of interbreeding. 



DIFFERENCES OF DEFICIENT ACCOMMODATION (TOPISM). 



Environmental differences are not all of one kind. Some of 

 them are the results of the power of accommodation or adjust- 

 ment (artism), while others represent rather a deficiency in 

 ability of this kind, so that the organism, though perhaps able 

 to maintain an existence, fails to attain one or another of the 

 normal characters of the species. Thus there is a variety of 

 canary bird which if fed on cayenne pepper during its period of 

 moulting produces red feathers instead of yellow. 



The South American Indians are said to be able to alter the 

 color of the feathers of their domesticated parrots by inoculating 

 them with the blood of toads. The colors of certain flowers 

 can be modified by special conditions or by treatment with 

 chemicals. The injury of the white pigs from paint-root, while 

 black pigs escaped, as related by Darwin, would be another 

 example of the same group of phenomena. 



The relations of topism to artism and to teratism are some- 

 times very intimate. A character assumed by one plant as a 

 means of accommodation may appear in another as a limitation 

 of the power of accommodation or as a complete abnormality. 

 The need of discrimination and the difficulty of exercising it 



