It has the appearance of being upon a path which species have a natural tendency to follow, a line of evolution which is perhaps rendered possible by constantly occurring small variations common to all animals and taken advantage of only under certain circumstances of pressure or increase. It seems not to depend on any sudden large variation of type, and such is not necessary to account for it. It differs from many other modifications which we know animal life to have undergone in being immediately useful to the species from its very beginning and in its least perfect forms. Once started, however imperfectly, the new habit will have a natural tendency to progress towards fuller forms of sociality by reason of special selective forces which it inevitably sets going. The fact that it is valuable to the species in which it develops even in its most larval forms, {103} combined with its tendency to progress, no doubt accounts for the wonderful series of all degrees of gregariousness which the field of natural history presents.
I have pointed out