GROWTH AND DEVELOPMENT OF THE INFLORESCENCE AND FLOWER 517 



but these phenomena have still to be brought into some acceptable 

 relationship with the primary facts of morphogenesis. 



Genetical investigations. In the preceding sections, the action of 

 various environmental and physiological factors and developmental 

 relationships, which appear to be of general occurrence in infloresence 

 and floral morphogenesis, have been noted. These common factors and 

 relationships, which are purely extrinsic, or are not closely and specifi- 

 cally genetical in character, seem likely to be involved in some aspects 

 of parallel development. It is, however, to the genetical constitution 

 of the individual species, and to difiFerences in genetical constitution 

 among species, that the distinctive floral morphology of a particular 

 species, and the great diversity in floral morphology in species at large, 

 must be primarily attributed. So we have to inquire how, and to what 

 extent, homologies of floral organization can be explained genetically. 

 In practice, the respective actions of extrinsic and intrinsic factors can 

 never be completely separated, but there are good reasons why we 

 should try to obtain fuller information on the specific effects of the 

 two categories of factors. 



In a genetic approach to the evolutionary differentiation of the 

 families of flowering plants, Stebbins (1950) considered that pheno- 

 typic modifications in species are referable to mutations of genes, the 

 genes' time of action, growth substances, allometric growth, and en- 

 vironmental factors, and that the basic questions related to the forces 

 that induce and establish mutations in natural populations. The prob- 

 able random nature of individual mutations, the accumulation of those 

 that have a relatively slight effect on the phenotype, and the selective 

 advantage conferred on individuals and populations by combinations 

 of different gene-dependent properties, are well-established concepts 

 in contemporary genetics. Yet, as many biologists have recognized, all 

 long-continued evolutionary trends appear to be governed by some 

 guidmg force. While natural selection is now generally accepted as an 

 ever-present and effective agent, Stebbins, in common with some other 

 biologists, also considers that there is "some unexplained force which, 

 presumably by causing the more frequent or predominant occurrence 

 of mutations which are genetically unconnected, but have a similar 

 morphological and physiological effect on the phenotype, directs or 

 canalises the course of evolution." Whatever the nature of this "force" 

 may be, it is evidently very important and deserves to be more fully 

 explored. 



The details of Stebbins' explanation of the inception of different 

 systematic groups need not be considered here, but some points may be 

 noted. He emphasizes that since the flower is a harmoniously devel- 

 oped and functionally effective organ, any viable change in one of its 

 parts will "immediately change the selective value of modifications in 



