STAGES IN THE DEVELOPMENT OF SIUM 



CICUTAEFOLIUM. 



BY GEORGE HARRISON SHULL. 



The juvenile forms of leaves have been believed to be so related to the 

 evolutionary history of a plant as to indicate the form of leaf possessed 

 by its ancestors and to furnish satisfactory evidence of the closeness of 

 relationship between allied species. It is impossible, in most cases, to 

 determine the ancestors of any species, and it is likewise impossible, 

 therefore, to demonstrate a close parallel between ontogeny and phy- 

 logeny. As there is in both processes the development from some simple 

 condition to one and the same complex condition, namely, the climax 

 type of leaf of the present adult plant, we can scarcely escape the belief 

 that such a parallel does exist in many cases ; but how safely or in how 

 minute detail we may reason from ontogeny to phylogeny may well be 

 considered an open question. 



The hypothesis of von Baer (1828) has proved a very suggestive one, 

 and, like most suggestive hypotheses, has been given a much wider ap- 

 plication than its author would have been willing to sanction. Von Baer 

 did not assume that the adult characters of the ancestors occur as larval 

 or juvenile characters in the descendants, but that the same larval stages 

 occur in both, a given stage appearing earlier in the descendant than in 

 the ancestor. The idea that larval and juvenile characters agree with 

 ancestral adult characters was an old conception which was rehabilitated 

 by Louis Agassiz (1848-1849) and became crystallized in the epigram 

 of Haeckel (1866), which is now universally known as the "Law of 

 biogensis " that " ontogeny repeats phylogeny."* 



Great stress was laid upon this hypothesis by Hyatt, Cope, and others 

 who have used it as a most important principle in disentangling difficult 

 phylogenetic problems. More recently Jackson (1899) has called atten- 

 tion to the fact that in organisms having periodically interrupted 



*For a history of the development of the idea of 'repetition see Hyatt, A., 

 " Cycle in the life of the indvidual and in the evolution of its own group." The 

 law of repetition is there called " Agassiz's law of palingenesis." See also 

 Glaser, O. C., " The law of von Baer." 



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