_Unlike commensals._--The case of a community consisting of individuals belonging to one species is, strictly speaking, scarcely ever met with; but the dominant individuals of a community may belong to a single species, as in the case of a beech forest, spruce forest, or ling heath--and only thus far does the case proceed. In general, many species grow side by side, and many different growth-forms and types of symbiosis, in the extended sense, are found collected in a community. For even when one species occupies an area as completely as the nature of the soil will permit, other species can find room and can grow between its individuals; in fact, if the soil is to be completely covered the vegetation must necessarily always be heterogeneous. The greatest aggregate of existence arises where the greatest diversity prevails. The kind of communal life resulting will depend upon the nature of the demands made by the species in regard to conditions of life. As in human communities, so in this case, the _struggle between the like_ is the _most severe_, that is, between the species making more or less the same demands and wanting the same dishes from the common table. In a tropical mixed forest there are hundreds of species of trees growing together in such profuse variety that the eye can scarce see at one time two individuals of the same species, yet all of them undoubtedly represent tolerable uniformity in the demands they make as regards conditions of life, and in so far they are alike. And among them a severe competition for food must be taking place. In those cases in which certain species readily grow in each other's company--and cases of this kind are familiar to florists--when, for instance, Isoetes, Lobelia Dortmanna, and Litorella lacustris occur together--the common demands made as regards external conditions obviously form the bond that unites them. Between such species a competitive struggle must take place. Which of the species shall be represented by the greatest number of individuals certainly often depends upon casual conditions, a slight change in one direction or the other doubtless often playing a decisive rôle; but apart from this it appears that morphological and biological features, for example, development at a different season, may change the nature of the competition. Yet there are in every plant-community numerous species which _differ widely_ in the demands they make for light, heat, nutriment, and so on. Between such species there is less competition, the greater the disparity in their wants; the case is quite conceivable in which the _one species should require exactly what the other would avoid_; the two species would then be complementary to one another in their occupation and utilization of the same soil.