as _Entozoa_, living within the bodies of their hosts, now in parts of their alimentary canals, now on other of their mucous surfaces, and now in various of their organs: these last two groups being so numerous in their kinds that there are commonly more species than one proper to each larger animal. One stage further in the complication meets us in the parasites upon parasites. But now the general fact, to which these brief indications are introductory, is that the use made of one organism by another has been ever widening and becoming more involved. Among plants utilization of the larger by the smaller--of trees by epiphytes and parasites--must have arisen since the times when the larger came into existence--times relatively late in the course of organic evolution. Moreover most of the plants which utilize others, either by climbing up them or settling themselves high up on their stems or sucking their juices, are phænogams, and the plants they utilize are also phænogams; so that these innumerable interdependences must have been established since the phænogamic type has become so predominant in respect of both size and kind. Similarly among animals. Though there are many parasites belonging, like the Trematodes, to very low classes, there are many which belong to the _Arthropoda_, and, being degraded forms of that class, must have come into existence after Arthropods of considerable structure had been evolved. Again, a large part of the animals infested by _Epizoa_ and _Entozoa_ are vertebrates--many of the highest types; and as these are relatively modern all this parasitism must be of late date. So, too, of much commensalism and many mutually-beneficial associations. The reciprocal services of ants and aphides must have originated since the _Hymenoptera_ and _Hemiptera_ became established types, and since the days when certain insects of the ant-type had become social, and since the days when aphides had become degraded members of their order: both dates being relatively recent. And still more recent must have been the commensalism between the ants and the many species of other insects which inhabit their nests. Leaving out relations of the kinds just named, it seems that down from those between carnivores and their prey to those between lice and their hosts, such relations profit one of the two species concerned and injure the other, and that there the matter ends. But it does not end there; for that multiplication of effects to which people are usually blind, brings about changes which, as hinted above, though injurious to the individual are beneficial to the species, and which, when not beneficial to the species, are often beneficial to the aggregate of species.