41 8 THE GERM-PLASM 



question, and the co-operation of all of these determines the 

 character. A hereditary individual variation will therefore 

 arise if viany of the homologous determinants vary in the same 

 •way. 



We can thus understand the process of doubling of the 

 determinants, which must have occurred repeatedly, as more 

 complicated structures arose in phylogeny. More abundant 

 nourishment would cause a determinant to grow and multiply 

 more rapidly, and if the first multiplication occurs before the 

 ' reserve germ-plasm ' for the next generation has become 

 separated off, this double determinant will be permanently 

 retained by the species. The corresponding part of the body, 

 however, will only display a perceptible variation when the 

 majority of the homologous determinants have become doubled. 



Minute fluctuations must thus continually occur in the com- 

 position of the biophors and determinants. Their variability 

 depends on the same principle as the systematic disintegration 

 of the determinants in the germ-plasm, and is due to the 

 dissimilar composition of the elements of the growing substance. 

 If the determinants consisted of masses which were all exactly 

 alike, inequality of nutrition could never transform determinant 

 A into A^ r — it could only alter its rate of growth. They are, 

 however, corhposed of biophors of diiferent kinds, which react 

 dissimilarly under different conditions of growth. This renders 

 possible a disarrangement of the proportional numbers of the 

 different biophors in a determinant, and consequently also the 

 variation of the latter. It is therefore quite conceivable that all 

 the qualities of a cell are not affected by these influences, but 

 that only certain of them vary, and that only a few amongst a 

 large number of similar determinants need become modified. 



The facts already mentioned with regard to the climatic 

 variation of butterflies^ prove that such an alteration can actually 

 occur. In these insects the determinants of certain coloured 

 wing-scales are slowly changed in the course of generations, in 

 consequence of the rise in temperature of the climate, and thus 

 the colour of the scales becomes considerably modified. Such 

 cases of conspicuous variation are not often met with ; all species 

 of butterflies, at any rate, are not affected in this way by changes 

 of temperature, and those that are thus influenced do not display 

 the variation in all the different kinds of scales. This indicates 

 that the determinants have a strong tendency to remain constant. 



