SHARE OF THE PARENTS IN BUILDING UP THE OFFSPRING 51 



have varied^ and above all we must not take for granted that 

 the determinants of different characters or parts of the body, for 

 instance, the determinants L, M, or lY, must all undergo variation 

 in an equal number of ids. It will depend on two factors whether 

 a new character is implicit, in the form of varied determinants, in 

 a small or in a very large majority of ids : first, on the relative age 

 of the character, and, secondly, on its value in relation to the 

 persistence of the species. The more important a character is for 

 the species the more frequently is it decisive for the life or death 

 of the individual, and the more sharply will individuals not possessing 

 it be eliminated, and the more rapidly, therefore, will those whose 

 germ-plasm still contains a majority of the unvaried determinants 

 of this character tend to disappear. In this way these determinants 

 will tend to sink down from generation to generation to an ever 

 smaller minority in the germ-plasm of those that survive. 



Thus in the ids of any species which has been in some way 

 transformed — and that is as much as to say, in every species — the 

 equivalent or homologous determinants are modified in a very varied 

 percentage. A very modern and at the same time not very important 

 character IC will only be contained in a small majority of ids, while 

 in the remainder the original homologous ancestral determinant K 

 is contained ; an older but not very much more important character 

 M' must have its determinants in a larger majority in the ids, while 

 a character V^ of decisive importance for the preservation of the 

 species, if it has been in existence long enough, must be represented 

 in almost all the ids, so that the homologous unvaried determinants 

 of the ancestral species V can only have persisted in an id here 

 and there. 



If this argument be correct, many phenomena of inheritance 

 become intelligible, especially the variability in the expression of 

 the inheritance in the parts of the offspring, which is more or less 

 rigidly predetermined at fertilization. For the germ-plasm thus' 

 contains in advance every kind of determinant in diverse nuances, 

 and in definite numerical proportions. In a plant N', for instance, 

 Ba' may be the determinant of the modern leaf-form, and may 

 occur in twenty-two out of twenty-four ids of the germ-plasm, 

 while the two remaining ids still contain unmodified the old leaf- 

 form determinants Ba, which the ancestral form N possessed. But 

 the flower of N' may be of still more recent origin, and contain the 



^ By equivalent or homologous determinants I mean the determinants of different 

 ids which determine similar parts, e, g. the scales of that wing-spot in Lyccena agesiis 

 which is alluded to above, and to which we must refer again in more detail. 



E 2 



