The Theory of Natural Selection. 269 



sporadically, extend beyond the species. In these 

 latter cases members of different species mutually 

 assist one another, whether in the way of what is 

 called symbiosis, or in a variety of other ways which 

 I need not wait to mention. For the only point 

 which I now desire to make clear is, that all cases of 

 mutual aid or co-operation, whether within or beyond 

 the limits of species, are cases which fall under the 

 explanatory sweep of the Darwinian theory \ 



Another important point to notice is, that it con- 

 stitutes no part of the theory of natural selection to 

 suppose that survival of the fittest must invariably 

 lead to improvement of type, in the sense of superior 

 organization. On the contrary, if from change of 

 habits or conditions of life an organic type ceases to 

 have any use for previously useful organs, natural 

 selection will not only allow these organs in succes- 

 sive generations to deteriorate by no longer placing 

 any selective premium upon their maintenance but 

 may even proceed to assist the agencies engaged in 

 their destruction. For, being now useless, they may 

 become even deleterious, by absorbing nutriment, 

 causing weight, occupying space, &c., without con- 

 ferring any compensating benefit. Thus we can under- 

 stand why it is that parasites, for example, present the 

 phenomena of what is called degeneration, \. e. showing 

 by their whole structure that they have descended from 

 a possibly very much higher type of organization than 

 that which they now exhibit. Having for innumerable 



1 Prince Kropotkin in the Nineteenth Century (Feb. 1888, Apr. 1891) 

 has adduced a large and interesting body of facts, showing the great 

 prevalence of the principle of co-operation in organic nature. 



