128 THE EVOLUTION THEORY 



one or other of the descendants, and would thus break the previously 

 stable type of the most perfectly adapted species. The descendant in 

 question would then succunib in the struggle for existence. But as 

 the number of the determinants in the germ-plasm is probably much 

 o-reater than that of the descendants of one generation, every descendant 

 would in the course of time deviate unfavourably in some one character 

 from the type of the species, and then either all the descendants would be 

 eliminated or the type would become unstable. But neither of these 

 things happens, and there are undoubtedly species which remain 

 constant for long periods of time, therefore the assumption must be 

 false and every variation of a determinant does not of necessity 

 2fO on in the same direction without limit. 



I therefore suppose that although slight variations are ceaselessly 

 taking place upwards or downwards in all determinants, even in 

 constant species, the majority of these turn again in the other direction 

 before they have attained to any important degree of increase, at 

 least in the germ-plasm of all species which have had a definitely 

 established equilibrium for thousands of generations. In such a germ- 

 plasm, or to speak more precisely, in the id of such a germ-plasm, 

 marked fluctuations in the nutritive stream will not be likely to 

 occur as long as the external conditions are unchanged, but slight 

 fluctuations, which will not be w^anting even here, may often alternate 

 and turn in an opposite direction, and thus the upward movement of 

 a determinant may be transformed into a downwaid one. Every 

 determinant is surrounded by several others, and we can imagine that 

 the regular nutritive stream wdiich we have assumed may be partially 

 dammed up by a slight enlargement of the determinant, and that this 

 will drive the surplus back again. But however Ave may picture 

 these conditions, which are for all time outside of the sphere of obser- 

 vation, the assumption of a self-regulation of the germ-plasm, up to 

 a certain degree, cannot be regarded as inconceivable or unphysio- 

 logical. 



But there are limits to this self-regulation ; as soon as the 

 increase or decrease of a determinant attains a certain degree, as soon 

 as it has got beyond the flrst slight deviation, it overcomes all 

 obstacles, and goes on increasing in the direction in which it has 

 started. Tliis must happen even in the case of old and constant 

 species, and frequently enough to admit of an apparent capacity for 

 adaptation in all directions. Every part of a species can vary beyond 

 the usual individual fluctuations, and as this is possible only by means 

 of intra-germinal processes, we must assume tliat even in tlie case of 

 germ-plasms wliich have long remained in a state of stable equili- 



