374 THE EVOLUTION THEORY 



which the individual determinants have each their definite place. The 

 position of the determinants in relation to one another cannot be due to 

 chance, but depends partly on their historical development from earlier 

 ancestral determinants, partly on internal forces, such as we have 

 already assumed for keeping the determinants together. We may 

 best designate these hypothetical forces ' afiBnities,' and in order to 

 distinguish them . from mere chemical affinities we may call them 

 'vital.' There must be forces interacting among the different deter- 

 minants which bind them together into a living whole, the id, which 

 can assimilate, grow, and multiply by division, in the same manner as 

 we were forced to assume for the smaller units, the biophors and 

 single, determinants. In the ids, however, we can observe the working 

 of these forces quite directly, since each chromosome splits into two 

 halves of equal size at every nuclear division, and not through the 

 agency of external forces, e.g. the attraction which we' may assume to 

 be exerted by the fibrils of the nuclear spindle, but through purely 

 internal forces, often long before the nuclear spindle has been formed 

 at all. 



But if the determinants must separate from each other in the 

 course of development so as to penetrate singly into the cells they are 

 to control, the id must not only have the power of dividing into 

 daughter-ids of identical composition, it must also possess the power 

 of dividing under certain influences into dissimilar halves, so that the 

 two daughter-ids contain different complexes of determinants. The 

 first mode of division of the id, and with it of the nucleus and of the 

 cell, I call erbgleich, or integral, the second erhungleich, or differen- 

 tial. The first form of multiplication is the usual one, which we 

 observe everywhere when unicellular organisms divide themselves 

 into two equal daughter-units, or when the cells of multicellular 

 bodies produce their like by division into two. The second is not 

 directly observable, because a dissimilarity of the daughter-cells, as 

 long as it lies only in the idioplasm, cannot be actually seen; it can 

 only be inferred from the different role which the two daughter-cells 

 play in the building up of the individual. When, for instance, one of 

 two sister-cells of the embryo gives rise to the cells of the alimentary 

 canal and the other to those of the skin and the nervous system, I 

 infer that the mother-cell divided its nuclear substance in a diflBren- 

 tial way between the two daughter-cells, so that one contained the 

 determinants of the endoderm, the other those of the ectoderm; or 

 when a red and a black spot lie side by side and under exactly the 

 same conditions on the wing of a butterfly, I conclude that the 

 ancestral cells of these two spots have divided differentially, so that 



