GERMINAL SELECTION 153 



and thereby vary in their qualities. To follow this ont in greater 

 detail and attempt to guess at the play of forces within the minutest 

 life-complexes would at present only be giving the rein to imagination, 

 but in principle no objection can be made to the assumption that 

 every element of life down to the very lowest and smallest can, by 

 reason of inequalities in its nutrition, be not only started on an 

 ascending or descending movement of uniform growth, but can also 

 be caused to vary qualitatively, that is, in its characters, because its 

 component parts change their proportions. 



Of course we know nothing definite or precise with regard to the 

 units of the germ-plasm, and we cannot tell what is necessary in order 

 that a determinant shall determine a part of the developing body 

 in this way or in that : thus we have no definite idea of the relations 

 subsisting between the variations of the determinants and those of 

 their determinates, but we know at least so much, that hereditary 

 variation of a part is only possible when a corresponding particle 

 in the germ-plasm varies; and we may at least assume that these 

 correspond to each other so far, that a greater development of the 

 one implies a greater development of the other, and that a reversal 

 of these relations is impossible. If the determinant X disappears 

 from the germ-plasm the determinate X' disappears from the soma. 

 It is therefore justifiable to infer from the degree of development 

 of an organ the strength of its determinant, and to assume that plus- 

 and minus-variations in both are correspondingly large. 



But in addition to the fiuctuations in the equilibrium of the 

 germ-plasm which lie at the root of all hereditary variation, we have 

 to take into account something which we have already touched upon 

 briefly — the correlation of the determinants, the influencing of one 

 determinant b}^ those round about it. I have spoken for the sake 

 of brevity of 'the determinant' of a part, although all the large and 

 more important parts must certainly be thought of as represented 

 by several or many, if not, indeed, by whole groups of determinants. 

 Although it is quite out of our power to follow the complex processes 

 of the mutual influences of the determinants upon each other, we can 

 say this at least, that these influences must exist, and we have here 

 a faint indication of what must occur in the case of spontaneous 

 variations within the germ-plasm. We must, in the first place, think 

 of the individual determinants as arranged in groups, so that, for 

 instance, the determinants of the right and left half of the body lie 

 together, and therefore are frequentl}^ aftected together by influences 

 which cause variation, so that Ijoth vary in the same direction at 

 the same time. In point of fact, analogous deformities, such as 



