CONFORMITY TO TYPE 253 



cannot become essentially modified by subsequent influences. 

 The facts relating to identical twins and to plant hybrids prove 

 that this is so. 



"Reversion to grandparents and great-grandparents or to 

 uncles and aunts may be accounted for by the fact that, in the 

 first place, the idants and the ids are not formed anew in the germ 

 plasm of the parents, but are derived from the grandparents; and, 

 secondly, that the combination of ids contained in the individual 

 germ cells of the parents become very diversified in consequence 

 of the 'reducing division/ 



"The number of ids of any particular ancestor which are con- 

 tained in the germ plasm of a ripe germ cell depends entirely on the 

 manner in which the reducing division occurs; and, under certain 

 circumstances, a germ cell might presumably contain half the 

 entire number of ids of one grandparent and none of those of the 

 other three. The larger the number of ids derived from an 

 ancestor, the greater is the probability that some of the characters 

 of this ancestor will appear in the descendants; but this depends 

 on the force of the ids of the other parent which comes into play 

 when amphimixis takes place, and also on whether the ids derived 

 from this ancestor are the dominant ones which determine his 

 'type.' 



"From this theory it could be predicted that hybrid plants 

 fertilized with their own pollen must produce very variable off- 

 spring, and that individuals of these hybrids must, moreover, 

 revert to one or other of the ancestral species; both these state- 

 ments are borne out by fact. 



"The more remote the ancestors to the characters of which 

 reversion occurs, the more rarely will it take place. Reversion to 

 the three-toed ancestors of the horse, for instance, is of extremely 

 rare occurrence for it is due to the retention of ancestral determin- 

 ants which have certainly disappeared from all the ids in the germ 

 plasm of most existing horses. 



" The remarkable phenomenon of dimorphism, which has been so 

 extensively introduced more especially into the animal kingdom 

 by means of sexual reproduction must be due to the presence 

 in the idioplasm of double determinants for all those cells, groups 

 of cells and entire organisms which are capable of taking on a male 

 and female form. But only one half of such a double determinant 

 remains inactive, while the other remains active. The sexual 

 differentiation of the germ cells must thus be due to the presence 

 of Spermatogenetic and Oogenetic double determinants; and even 

 all the secondary sexual characters must be traced to a similar 

 origin in the idioplasm. 



"The assumption of double determinants is also able to throw 

 some light upon certain enigmatical phenomena of heredity 

 exhibited by human beings. It has long been known that hemo- 

 philia (the bleeder's disease) occurs in men only, but is trans- 

 mitted by women. This disease, like a secondary sexual character, 

 is only transmitted to the sex in which it first appeared, for this 

 half of the double determinants of the 'mesoblast germ ' has alone been 

 modified by the disease. 



"It is self-evident from the theory of heredity here propounded 

 that only those characters are transmissible which have been 



