1 18 MORPHOLOGICAL DEVELOPMENT. 



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 ; but 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 effect of the modifying causes accumulates. That 

 it may accumulate the flowers must keep themselves so re 

 lated 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 uni 

 formity in the relations of their parts to surrounding influences, 

 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 inclinations 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 off 

 spring by some opposite functionally-produced modification 

 in another flower. It is quite comprehensible, therefore, 

 that irregularly-branched plants should thus preserve their 

 laterally-borne flowers from undergoing permanent devia 

 tions from their primitive radial symmetry. Fig. 230, re- 



Presenting a blossoming 

 twig of the Blackthorn, 

 will illustrate this. 

 Again, upright panicles 

 such as that of the 

 Saxifrage, shown in Fig. 

 231, and irregular terminal groups of flowers otherwise 

 named, furnish conditions under which there is similarly an 



