292 ME. A. S. HOENE — ^A CONTRIBUTION TO 



As the incidence of the ' Origin of Species ' shed transcendent light upon the 

 mystery of the Natural System in 1859, so the work of these great protagonists in 

 experimental evolution, and the expositions emanating from the schools of experiment 

 and thought founded hy them, cannot fail in time to illuminate Systematic Botany 

 and inaugurate a new phase in the evolution of the idea of the Natural System. 



De Vries hy a rigid analysis of G^notJiera separated numerous diverse forms breeding 

 true, some exhibiting comparatively great differences. ^J'he Mutation theory postulates 

 the evolution of forms progressively divergent. Hence it is conceivable that forms — for 

 instance, genera grouped in different families — are in some cases comparatively closely 

 related. Again, the characters exhibited by De Vries's mutant (Enotheras are paralleled in 

 other mutating races — for example, giantism in (Enothera gigas also occurs in Lathyrus 

 odoratus (Bateson, 1909), Pisum sativmn (Mendel), and in Primula sinensis (Keeble, 

 1912). Pacts of this kind serve to redirect attention to the importance of the doctrines 

 of parallel development and convergence in relation to floral evolution, especially since 



o 



atural races trend in the direction of synthetic and economic 



Convergence in reproductive characters may occur even more frequently than has yet 

 been contemplated, so that like forms, such as genera in the same family, may be less 

 closely related than others are that differ more pronouncedly. Hence the Principle of 

 Hesemblances must now be qualified by a greater regard for the significance of differences, 

 even if it be not shaken to its foundation by the doctrine of convergence and the 

 existence of insidious parallelisms. 



Before proceeding to attempt to solve the phylogenies of the families considered here, 

 it will be convenient to review the evolutionary processes that have been discussed in 

 relation to the results obtained by the experimentalists. But the prospect is by no 

 means encouraging, since De Vries * write«? : *' The great lines of evolution of whole 

 families and even of genera and of many larger species obviously lie outside the limits 

 of experimental observation. They are the outcome of the history of the ancestors of 

 the present types, and a repetition of their history is far beyond human powers." 



The carpels in Davidia vary from five to ten, with a mean number of six or seven ; 

 they vary in number in the forms of Iledera Helix ; and, in Samhucus, species are 

 characterized by five, and a range of from two to four with a mean of three respectively. 



Mutation in the carpels has not yet been specially mentioned as occurring in the races 

 produced by experiment. 



Syncarpy supervenes upon apocarpy,and the advance towards syncarpy is accompanied 

 or followed by that towards perigyny and epigyny, but here again there is as yet little 

 direct experimental evidence. 



The most fascinating phenomena are those attendant upon sterilization. A vast 

 number of instances are known in which sterility is probably due to seasonal conditions, 

 or to diversion or poorness of the nutrient supply, including the very numerous cases 

 where only few of very many flowers develop in the inflorescence. Davidia, where all 

 il^Q ovules in the ovary may be fertile or sterile, may be included in this category. 

 Thence we pass to numerous other cases in which it is doubtful whether sterility has 



• 



H. De Vries, ' Species and Varieties ' (1905) p. 223. 



