92 ROBERT TRACY JACKSON ON 



or less extensive. It may be reduced to a single part, as a leaf or even a portion of a leaf 

 of a plant, or a plate of an echinoderni ; or it may include a series of parts in which local- 

 ized stages are observable, as in suckers of plants, certain portions of a branch (Larch) , 

 and diseased, i-eversionary, or otherwise modified branches of plants (Red Cedar, Tulip- 

 tree) . It is obvious that this latter distinction is only a modification of the first, but yet 

 is convenient to maintain. 



Localized stages may be developed in the natural growth of the individual, or they 

 may be evinced on\y under more or less abnormal or pathologic conditions. Cases of this 

 are reversionary suckers in the Pitch Pine, suckers developed on the felling of trees, 

 growths on mutilated branches of Liriodendron and Ailanthns, tuft-like branches of the 

 Red Cedar with acicular leaves. 



Youthful or reversionary features may be developed only at certain definite areas of 

 the organism. Such are compound leaves at the base of AmpeJojJsis tricusjndata, ovate 

 leaves on the cui-rent season's growth of the same, arrangement of leaves on the current 

 season's growth in the Larch. In general, suckers from the base of the trunk or roots of 

 trees have a strong tendency to reproduce nepionic or reversionary characters. 



If there is truth in the principle of localized stages in development, from them we 

 should be able to predicate with some accuracy the conditions of the j'oung, and concur- 

 rently of ancestral types as well. I think that this may be done. The existence of local- 

 ized stages was first observed in Palaeozoic Echini and was published in Studies of 

 Palaeechinoidea ('96, p. 228). 



The occurrence of locahzed stages, and their bearing, may be expressed in the follow- 

 ing law, which should be compared with the laws concerning youthful and senile stages : 

 Throughoift the life of the mdividual, stages may he found in localized jiarts, ivhich are 

 similar to stages found in the young, and the equivalents of which are to be sought in the 

 adults of ancestral groups. While this law covers the usual conditions, it is possible and 

 even probable that degradational or progressive features may appear as localized stages. 

 To include such cases the following clause may be added : The equivalents of regressive 

 or progressive localized stages are to he sought in the adults of degradational or progres- 

 sive series of the group. 



My sincere thanks are due to Mr. Charles E. Faxon of the Arnold arboretum. Harvard 

 university, for reading the manuscript of the botanical portion of this paper, and for help- 

 ful suggestions on the same, also for aiding me with material. For seedhngs and other 

 material, studied at the Arnold arboretum, my thanks are due to Prof. Charles S. Sargent, 

 Director of the Arboretum. For many seedlings valuable in my studies I am indebted to 

 the kind interest of Miss Marian C. Jackson, the Shady Hill nurseries of Bedford, Mass., 

 and others as mentioned in the text. 



