DEVELOPMENT OF SIUM CICUTAEFOLIUM. 9 



development. If these juvenile leaves do correspond with adult condi- 

 tions of the past we might suppose that at the time when the species was 

 characterized by a tripartite leaf it was much more variable than it be- 

 came later, and that the process of evolution acted to give it gradually a 

 more and more fixed character. Or, on the other hand, it might be sup- 

 posed that the species has been at all times about equally variable, but 

 that by the " law of acceleration " the first nepionic leaf represents a 

 much longer period in the history of the species than does any subse- 

 quent leaf. Again, it may be that these leaf-forms are not atavistic, and 



Fig. 1 .—Graphic representation of the variation In the several " nepionic " 

 leaves of Sium cicutae/olium, from the first to the eighth, inclusive. 

 Ordlnates given in per cents. 



the wider range of variation may merely represent a greater sensitive- 

 ness to minute variations of the internal and external environment. As 

 the plant becomes more and more firmly established it becomes morpho- 

 logically more s6lf-determinative. In other words, the greater variabil- 

 ity and the gradual decrease in variability may be purely physiological 

 facts unrelated in any direct way to the phylogenetic history of the 

 species. It is possible indeed that each of these propositions mat rep- 

 resent a partial truth, as they are not mutually exclusive. The species 

 may have been more variable in its adult leaf-form, the law of accelera- 

 tion may give rise to a greater range of atavistic variation in the earlier 



