THE SHAPES OP FLOWERS. 163 



it to continue wherever there are no forces tending to change 

 it. What now must be the forces tending to change it? 

 They must be forces which do not simply affect differently 

 the different parts of an individual flower. They must be 

 forces which affect in like contrasted ways the homologous 

 parts of other individual flowers, both on the same plant and 

 on surrounding plants of the same species. A permanent 

 modification can be expected only in cases where, by inherit- 

 ance, the effects of the modifying causes accumulate. That 

 they may accumulate the flowers must keep themselves so 

 related to the environment, that the homologous parts may, 

 generation after generation, be subjected to like differentiating 

 forces. Hence, among a plant's flowers which maintain no 

 uniformity in the relations of their parts to surrounding in- 

 fluences, the radial form will continue. Let us glance at the 

 several causes which entail this variability. When 



flowers are borne on many branches, which have all inclina- 

 tions from the vertical to the horizontal as are the flowers 

 of the Apple, the Plum, the Hawthorn they are placed in 

 countless different attitudes. Consequently, any spontaneous 

 variation in shape which might be advantageous were the 

 attitude constant, is not likely to be advantageous; and any 

 functionally-produced modification in one flower, is likely to 

 be neutralized in offspring by some opposite functionally-pro- 

 duced modification in another flower. It is quite compre- 

 hensible, therefore, that irregularly-branched plants should 

 thus preserve their laterally-borne flowers from under- 

 going permanent devia- 

 tions from their primi- 

 tive radial symmetry. 

 Fig. 230, representing a 

 blossoming twig of the 

 Blackthorn, illustrates 

 this. Again, upright 

 panicles, such as those of the Saxifrage exemplified in 

 Fig. 231, and irregular terminal groups of flowers other- 



