From what these forms have in common, let us turn to that which they have not in common, and observe the causes of the want of community. A puff-ball shows us in the simplest way, the likeness of parts accompanying likeness of conditions, along with the unlikeness of parts accompanying unlikeness of conditions. For while, if we cut vertically through its centre, we find a difference between top and bottom, if we cut horizontally through its centre, we find no differences among its several sides. Being, on the average of cases, similarly related to the environment all round, it remains the same all round. The radial symmetry of the mushroom and other vertically-growing fungi, illustrates this connexion of cause and effect still better. But now mark what happens in the group of _Agaricus noli-tangere_, shown in Fig. 195. Radially-symmetrical as is the type, and radially symmetrical as are those centrally-placed individuals which are equally crowded all round, we see that the peripheral individuals, dissimilarly circumstanced on their outer sides and on their sides next the group, have partially changed their radial symmetry into bilateral symmetry. It is no longer possible to make two corresponding halves by _any_ vertical plane cutting down through the pileus and the stem; but there is only _one_ vertical plane that will thus produce corresponding halves--the plane on the opposite sides of which the relations to the environment are alike. And then mark that the divergence from all-sided symmetry towards two-sided symmetry, here caused in the individual by special circumstances, is characteristic of the race where the habits of the race constantly involve two-sidedness of conditions. Besides being exemplified by such comparatively undifferentiated types as certain _Polypori_, Fig. 196, _a_, _b_, this truth is exemplified by members of the genus just named. In _Agaricus horizontalis_, Fig. 196, _c_, we have a departure from radial symmetry that is conspicuous only in the form of the stem. A more decided bilateralness exists in _A. subpalmatus_, shown in elevation at _d_ and in section at _d′ _. And _Lentinus flabelliformis_, of which _e_ and _e′_ are different views, exhibits complete bilateralness--a bilateralness in which there is the greatest likeness of the parts that are most similarly conditioned, and the greatest unlikeness of the parts that are most dissimilarly conditioned.