GENERAL CONCLUSIONS 327 



Psilotineae, Lycopodineae, Equisetineae and Filicineae are sufficiently 

 different as to be regarded as distinct phyletic lines, though the possi- 

 bility of their having sprung from a common ancestor, or ancestral 

 group, is not necessarily precluded. Similarly, a study of gymnosperm 

 and angiosperm embryos affords no clue as to how the latter should be 

 related phylogenetically to the former. 



Investigations of numerous dicotyledon embryos have shown with 

 what a high degree of precision and constancy the successive develop- 

 ments take place in any particular species. Particular genetical factors 

 apparently become effective early in the ontogenetic development. 

 If, now, in a species which is in an active state of evolution, there 

 are mutant genes which affect the early embryogeny as well as the 

 later stages of development, it is conceivable that the embryonic 

 development could become so modified that the older embryos of 

 parental and mutant types would no longer closely resemble each other. 

 Considerations such as these enable us to envisage, if only in a very 

 general and speculative way, how major systematic groups could have 

 originated from a common source. In some such way the divergence 

 of the bryophytes and the several lines of vascular plants from a 

 common protoarchegoniate ancestral stock could perhaps be explained. 

 Such views must, of course, be treated with caution: the prevalence 

 of parallel evolution must always be borne in mind and accorded due 

 consideration. 



The embryos of related organisms tend to be more alike than the 

 aduUs, and this Haldane (1932) has explained by saying that the genes 

 which determine the interspecific differences exercise their chief effects 

 rather late in the individual development. But if, as a result of 

 mutational change, certain genes became active at an early stage in the 

 embryogeny, far-reaching morphological and structural changes might 

 follow; and it might be difficult to realise that the mutant and the 

 parental forms were really closely allied. And if we further suppose 

 that changes of this kind, which have been described as acceleration, 

 together with other changes, have taken place cumulatively over a long 

 period of time, the eventual morphological differences, which are the 

 bases for separating the larger systematic units, may well mask the 

 original community of origin. The several effects on the embryogeny 

 and adult structure produced by changes in the time of action of genes 

 have been discussed in some detail for animal embryos by de Beer 

 (1930, 1940, 1951). The same process may simultaneously accelerate or 

 retard the action of a large number of genes. Thus a change in per- 

 meability, perhaps due to a single gene, would affect the action of many 

 gene-determined metabolites, with important consequential effects on 

 the morphological development. Until we gain a better understanding 



