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 fa!l 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 zmprovement 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, i.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 Mineteenth 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. 
