Domestic animals are all commensal, for without them civilized man cannot exist. The cat cannot exist without man, and it is tolerated for its benefits. The horse has been man's companion so long and undergone such tremendous artificial selection, that we really are doubtful as to what his wild ancestor was like. Our intense love for the horse is really self-love, he is part of us. No automobile, bicycle or any other contrivance for transportation can wipe out the joy of being on horseback. It is home - for there we evolved. If he went fast he allowed us to escape enemies or find food, and those with the love of fast horses survived by the natural selection of these lovers. The horse haters were all killed off in prehistory. Will any artificial, unnatural religion, therefore, ever be able to stop this sport? The horse gets just as much joy out of it as man, or he would not have evolved with man.
It is a strange side issue to this new idea in biology, that even the carnivorous or herbivorous enemies, which eat entire individuals of another species, are really beneficial in one sense. If they did not exist, their victims would increase beyond the food supply and die anyhow. Birds and insects which occasionally destroy vegetation are generally merely pruning the branches or scattering the seed or pollen. Darwin first showed the com-mensalism of clover and bees, the latter carrying the pollen to the female flowers from the male. What a strange outcome to modern biological discoveries, as to the preservation of the species being the first law. Our enemies are friends in disguise - gardners thinning out the garden to improve the rest, the very basis of evolution, for without enemies we would never have improved! It is now known that no group of plants or animals can be affected without affecting others. The relationships are so close and all are so interdependent that the destruction of one species may even cause destruction of the commensal organism. The whole living world, from man to bacteria, exists on a firm basis of commensalism. Whatever is, is good.
Prince Kropotkin has written a book on this one topic, showing that the struggle for existence is not to the strong always, but sometimes to the weak when they are the fittest for rendering service to the strong.* There has actually been a new phrase invented to cover these cases - "the utilization of the unfit" - that is, those unfit for independent existence.
Woods Hutchinson * writes of love as a factor in evolution, and deals at some length with the question of commensal relationships of organisms. What he calls love is, to a certain extent, the tie which binds together those organisms which depend upon one another, as in herds of one species, or associates of separate species. Self-sacrificing love for wife or offspring is but one form of commensalism, for in all cases of mutual association there must, of necessity, be some self-sacrifice. In other words, commensalism is based on mutual altruism. Pure selfishness (egoism) defeats the object and destroys the opposite organism. Every now and then the announcement is made that mutual aid in communities ends the individual struggle for existence. That is an error, for the struggle keeps up in other ways than murder.
Commensalism shows, of course, that any organism which cannot render assistance in return for services rendered it, is a burden which in time must destroy its benefactors, and all such must eliminate themselves of necessity. The obligation to return favors is the prerequisite for survival and is universal. The human body itself is formed of specialist cells in mutually beneficial relations. A few years ago, Prof. S. B. Laache, of Christiana, Norway, published an article on "Reciprocity in Pathology," in which it was shown that the various organs and parts of our body are also in commensal relationships, that is, mutually assisting each other, yet each dependent on the rest.
*"Mutual Aid, a Factor of Evolution".
* Monist, January, 1898.
Similarly, society is formed of specialist men or groups of men in similar dependence on society for survival and rendering aid to society in return.