THE GENERAL SHAPES OF PLANTS. 139 



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. Eadially symmetrical as is the type, and 

 radially symmetrical as are those centrally-placed individuals 

 which are equally crowded all round, we see that the peri- 

 pheral 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 pro- 

 duce 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, &, this 

 truth is exemplified by members of the genus just named. 

 In Agaricus Jiorizontalis, 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. sub- 

 palmatus, shown in elevation at d and in section at d'. And 



