124 



MORPHOLOGICAL DEVELOPMENT. 



centre, we find no differences among its several sides. 

 Being, on tlie average of cases, similarly related to the envi- 

 ronment all round, it remains the same all round. The 

 radial symmetry of the mushroom and other vertically- 



/S3 



growing fungi, illustrates this connexion of cause and 

 effect still better. But now mark what happens in the 

 group of Agaricus xylophilus, shown in Fig. 195. Radi- 

 ally symmetrical as is the t}^e, and radially sjonmetri- 

 cal as are those centrally-placed individuals which are 

 equally crowded all round, we see that the peripheral indi- 

 viduals, dissimilarly circumstanced on their outer sides and 

 on their sides next the group, have partially changed their 

 radial sjnnmetry 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 cor- 

 responding 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 sjrmmetry towards two- 

 sided symmetry, here caused in the individual by special 

 circumstances, is characteristic of the race where llie 

 habits of the race constantly involve two-sidedness of condi- 

 tions. Besides being exemplified by such comparatively 

 undifferentiated types as Boletus ^ Fig. 196, a, b, this truth 

 is exempKfied by members of the genus just named. In 

 Agaricus horizontalis, Fig. 196, c, we have a departure from 

 radial sjonmetry that is conspicuous only in the form of the 

 stem. A more decided bilateralness exists in A. palmatusi 



