106 Atavism. 



and with related phenomena. Goebel's admirable in- 

 vestigations have demonstrated the wide distribution of 

 these phenomena and their great importance to the theory 

 of descent.^ It is now a matter of common knowledge 

 that many plants, and indeed whole groups of species, 

 exhibit characters wdien young which they either lack in 

 the adult state, or which in later life appear only under 

 definite circumstances. Beissner's discovery- that whole 

 genera of cultivated Coni ferae, such as Rctinospova, are 

 only youth-forms of other known types such as Thuya; 

 and Reixke's investigations"' into the earlier stages of 

 Leguminosae, as well as the work of many others, have 

 resulted in the accumulation of a mass of information 

 relating to this subject. Siuui and Bcrula in their early 

 stages have the doubly pinnate and finely slit leaves of their 

 close allies ; the thorns of Bcrhcris on the so-called suck- 

 ers revert to the foliate form. These phenomena, how- 

 ever, fall mostly within the sphere of systematic botany, 

 and only concern the study of variability in so far as they 

 are dependent on external influences. 



\\t must further exclude from our considerations 

 the effects of crossing. The so-called reversions of the 

 horticulturists which are brought about either by acci- 

 dental crosses with the parent or by unconsciously using 

 hybrid seed, certainly occupy a very ])rominent place 

 in the practice of horticultural selection, but they should 

 be rigidly excluded from scientific speculations. And 



^ K. GoEBEL, Uchcr Jiigendformen von Pflanzcn und dcrcn kiinsf- 

 lichc WicdcrJicrvorrufung.. Sit7.iingsber. d. k. bayr. Akad. d. Wiss., 

 Vol. 26. i<S96, Part III. For further references see Goebel's Organo- 

 graphie der PHancen, Part I, 1898. 



" L. Beissner, Handbuch der Nadelholzkundc, 1891. 



^J. Retnke, Untcrsuchungcn i'lhcr die Assimilafioiisorgauc der 

 Leguuii)ioscn, I-III and IV-VII. Jahrbi'icher fiir wissensch. Botan., 

 Vol XXX, Parts i and 4, pp. i and 71, 1897. 



