of the chances of insect-fertilization unless the insect-proboscis in certain species lengthened _simultaneously_.[108] Cases also have been noticed of sea-anemones, which attach themselves to the shells of hermit crabs and by their poisonous tentacles repel attacks on the crab.[109] The crab is no doubt useful to its guest by providing it with the means of locomotion. Still, the case of mutual help between two such different orders of beings is remarkable. A very peculiar case is that of the waterfern, _Azolla_, which has certain roomy cavities on the underside of its leaves. These are always found to be occupied by a small unicellular organism of the _Alga_ order (_Anabæna_). It is of no apparent use to the _Azolla_, which provides it a home. The arrangement must have been of immensely long standing, for it occurs in all the four species of _Azolla_, one of which is found in America, two distributed over Australia, Asia, and Africa, and one only in the Nile. It must, therefore, have arisen before the original species split into four.[110] It would be rash to conclude from these and some similar curiosities that we are really in the presence of the phenomenon of disinterested aid given by one species to another. The question needs more investigation. But an important general consideration arises in this connexion. It is clear that there could be no advance in evolution if nature consisted solely of a multitude of independent units of life, fiercely competing against or warring with each other. It is equally clear that no advance could take place if every organism found an environment so perfectly adapted to it as to call for the very minimum of effort and strain in the maintenance of life. Between the chaos of the first supposition and the lubberland of the second there must be a condition of nature in which synthetic organization is carried just to the point at which life will have the maximum power to perfect and to realize itself. Looking at the conditions of nature as we know them, and at the majestic expression of material and spiritual life which those conditions have permitted, we may well be content to believe that both the synthetic process, as far as it goes, and its _apparent_ suspension at a certain point in the ascending scale, are the outcome of one and the same motive and have one and the same significance—they both alike mean and make for the conservation, the development, the enrichment of life.