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. 1 It is now a matter of common knowledge 
that many plants, and indeed whole groups of species, 
exhibit characters when young which they either lack in 
the adult state, or which in later life appear only under 
definite circumstances. BEISSNER'S discovery 2 that whole 
genera of cultivated Coniferae, such as Rctinospora, are 
only youth-forms of other known types such as Thuya; 
and REINKE'S investigations 3 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. Sium and Bcrnla in their early 
stages have the doubly pinnate and finely slit leaves of their 
close allies ; the thorns of Berberis 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. 
We 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 prominent place 
in the practice of horticultural selection, but they should 
be rigidly excluded from scientific speculations. And 
1 K. GOEBEL, Ucbcr Jugcndformen von Pflanzen und dercn kilnst- 
Viche Wiederhervorrufung. Sitzungsber. d. k. bayr. Akad. d. Wiss., 
Vol. 26, 1896, Part III. For further references see GOEBEI/S Organo- 
graphie dcr Pftanzcn, Part I, 1898. 
2 L. BEISSNER, Handbuch der Nadelhohkunde, 1891. 
3 J. REINKE, Untcrsiichungen fiber die Assimilationsorgane dcr 
Leguminoscn, I-III and IV-VII. Jahrbiicher fiir wissensch. Botan., 
Vol. XXX, Parts i and 4, pp. I and 71, 1897. 
