DEVELOPMENT OF SIUM CICUTAEFOLIUM. 



It has been claimed that the apex of a leaf presents primitive condi- 

 tions and that new leaf-characters appear at the base of the blade. It 

 is shown here that the base is also " primitive " and that the new char- 

 acters appear above the base. 



The proximal leaflets of pinnate leaves on the same stem are homolo- 

 gous, as are all other pairs of leaflets having positions of like order, 

 counting from the proximal pair. 



The sole basis for the assumption that localized stages present ata- 

 vistic characters is the fact of their greater simplicity. No satisfactory 

 inferences can be drawn from ontogenetic leaf-characters regarding 

 the phylogenetic history of the species. The various stages are the 

 result of present protoplasmic structure instead of the past history of 

 the protoplasm, and a change of structure which results in new adult 

 characters may also produce changed juvenile and senescent characters. 

 They are in need of physiological instead of phylogenetic interpretation. 



STATION FOR EXPERIMENTAL EVOLUTION, 



COLD SPRING HARBOR, N. Y., December, 1904. 



