The diminution of the death rate, which is the cause of the diminution of the birth rate, is itself a result of the great natural law of commensalism or mutual aid which is at the basis of all cooperation for survival. This law is part and parcel of the struggle for existence, that is, organisms which aid each other against a common enemy have a better chance for survival. Indeed, every combination depends upon the mutual assistance of the units and their mutual dependence, and as such unions began when the cells began to adhere, the law of mutual aid is as old as multicellular life, and as it is also the basis of the modern expansion of nations and their increasing dependence upon each other, it is necessary to have a clear conception of it before we can understand the present trend of events. It is bound to cause profound alterations in our form of government, and in the relationship of the lower and higher races, all of whom are under such intense expansive pressure.
The first organization, of course, was the family, and differed in no respect from that of lower mammals, but in time large bodies or clans were bound together, and the first nations were all blood relatives. Then larger and larger organizations survived by reason of their ability to destroy weaker competing clans. In this way nations have been growing larger and larger by the absorption or death of competitors, and the struggle for existence was changed from an individual fight to a collective one. Of course, no organism or nation can survive unless means are taken to preserve and protect each of the units. That is, life saving within the organization is absolutely necessary to enable it to destroy the men of competing clans or nations. Mutual aid within the nation is, there ore, essential, but the process is even more extended still, for it not infrequently happened that two competing tribes were compelled to form an alliance against a common enemy strong enough to destroy them separately. Such alliances of dissimilar peoples form the bulk of history, and find their counterpart throughout all nature.
Biologists have collected so many instances of two animals or plants of different species living together for their mutual benefit that we are now beginning to think that this phenomenon is universal. The vast majority were formerly called parasitism because one organism was small and subsisted on food obtained by the other, and no benefit to the larger could be shown. Investigation has brought to light this benefit in case after case, and caused us to transfer the organism to the commensal class. That there is mutual benefit in all cases is more than suspected from the universality of the association.
Jackals and hyenas dogging the footsteps of lions to eat the bones left over from the feast warn the lions of danger. Various "sucking fish" attach themselves to sharks, turtle, swordfish and whales as guides of some sort. Birds benefit large animals by eating their insect pests, the petrel on the whale, also a snipe called phalarope, carabao birds, cow birds, and so on indefinitely. Until the benefit was found, other names were invented, such as mutualism for those strange partnerships helpful in some way, like the partnership of blackbirds and fishhawks, the former building on sticks outside the nests of the latter and no doubt warning the hawk of danger or even protecting the whole nest during the long fishing absences of the hawk. Messmate is the term used for organisms which merely share food, but in which no benefit to either could be found, the weaker simply taking the crumbs from the rich man's table, like the chicken taken to town by the economical farmer to eat the grain dropped from the mouth of the horse. Anyhow, we know that the class of parasites is growing smaller as we learn more about them. The idea is also growing that every species has commensal species upon which it is wholly dependent for existence, and they must live side by side. Hence, it is not at all unlikely that every parasite will be found to bear some commensal relation to the host and the class of parasites disappear from our books. The word symbiosis is generally used by naturalists to cover cases of animals "living together" for their mutual benefit. We may here and there refer to organisms as symbiotics, but the word commensal is preferable, for though etymologically it merely means "eating at the same table," it has acquired an additional meaning of mutual aid.