402 ORGANIC GROWTH 



But this is to dispute heredity altogether, it is maintained 

 that the variety of plant -forms is due to external stimuli 

 only, which act upon plants during their development. 

 Whither does such a view lead? To the denial of the very 

 nature of embryonic development, which incontestably de-' 

 pends principally on heredity. It leads to the assumption 

 that all germs are equivalent, and that the fact that from a 

 cherry-stone a cherry-tree, from a pea a pea-plant arises, is 

 due only to the difference of external stimuli. 



In a work l in which he defends himself against Sachs's 

 attacks, Vochting uses language which even more distinctly 

 than the previous quotations shows the agreement between 

 his views and mine, an agreement which I value the more 

 because it was only after the preceding pages had been 

 written that I became accurately acquainted with the con- 

 tents of the work referred to. Amongst other things he 

 says that every organ, of whatever kind it may be, contains 

 internal determining conditions, and these in the cases re- 

 corded by him primarily determined at what points roots 

 should arise in a fragment of willow. Such internal condi- 

 tions " primarily determine the position of new formations. 

 The effects of the action of the external stimuli are thus 

 modified by internal factors." 



These " internal conditions " are the same as those which 

 I have called constitutional causes. 2 



Like Sachs, Pfliiger also denies heredity 3 and inasmuch 



1 Ueber Organbildung im Pflanzenreich, ii. p. 192, Bonn, 1884. 



2 Sachs has since (J. Sachs, Staff und Form, der Pflanzenorgane, ii., Arb. d. 

 botan. Inst. zu Wiirzburg, Bd. ii. p. 689), not only assumed that there are such in- 

 ternal causes, but even declared that he never denied their existence. It may be 

 remarked that Sachs (Naturwissenschaftliche Rundschau 1886, No. 5) also main- 

 tained that he had put forward the theory of the continuity of the germ-plasm 

 before Weismann (Sachs, Pfianzenphysiologie 1882, ch. xliii.) But as Weismann 

 replied (Zur Annahme einer Continuitat des Keimplasmas. Her. d. naturf. Ges. zu 

 Freiburg i. B. 1886), Sachs only referred to the fact that from the germ-sub- 

 stance of one generation that of the following is derived. 



3 Pfliiger, Ueber den Einftuss der Schwerkraft auf die Theilung der ZeUen 



