DEVELOPMENT OF SIUM CICUTAEFOLIUM. 25 



were characteristic of the whole adult leaf at some time in the past 

 history of the species is the fact of their greater simplicity. The rela- 

 tion is purely a logical one, and the condition of these regions of the leaf 

 are not unlikely to disagree with phylogenetic conditions in every detail 

 except simplicity. 



Although this study makes it evident that no satisfactory inferences 

 can be drawn from the forms occurring at various ontogenetic stages 

 regarding the ancestral adult conditions, these forms do have a bearing 

 upon the relationship of allied species. The similarity of two species 

 as to the characters possessed at any of these stages would lend evidence 

 in favor of their close relationship in the same way that similarity in 

 any other activity or character would, not because of resemblance to a 

 common ancestral adult condition, but because of the presumptive evi- 

 dence of similarity in present protoplasmic structure. 



All the evidence now available indicates that when specific differen- 

 tiation takes place the changed structure of the protoplasm which pro- 

 duces new adult characters also gives rise to new characters in the 

 juvenile and senescent series. The assumption that these stages have 

 a phylogenetic significance tends to obscure the fact that they are the 

 results of present conditions instead of the past history of the proto- 

 plasm and that they are in need of physiological rather than phylo- 

 genetic investigation and interpretation. 



