278 THE GERM-PLASM 



cies ; * but a predominance of paternal or maternal ids may 

 nevertheless exist, so that there may be a greater iiUDiber of 

 hottiodynanious determinants in one line of ancestors than in the 

 other, as has already been shown to be the case in hybrids. 



By the term honwlogous determinants, we understand those 

 elements of the idioplasm which are capable of so controlling 

 homologous parts of the body or determinates ; Jionwdyiiamoiis 

 determinants, on the other hand, are those of the homologous 

 determinants which have the special function of impressing a 

 like character on any part of the body, and which, taking an 

 example we have already made use of, serve to produce a 

 particular form and colour in a certain region of a butterfly's 

 wing. That parent in which the ids contain numerous homo- 

 dynamous determinants, or at any rate a larger number than 

 are contained in the ids of the other parent, must undoubtedly 

 exert the greater controlling power. The power of homo- 

 dynamous determinants is simply cumulative, whereas dissimilar 

 or heterodynamous determinants may, in the most favourable 

 cases, co-operate to form a single resultant, but may, under 

 certain circumstances, counteract or even neutralise one another. 

 The larger the number of homodynamous determinants which 

 the entire aggregate of ids of a parent contains at any ontogenetic 

 stage, the greater will be the likelihood that these will pre- 

 dominate in the struggle of the parts which takes place in the 

 cell, and will therefore stamp the latter more or less distinctly. 



The colour of the eyes may be taken as an example. Let us 

 take a case in which those of the mother are blue, and those of 

 the father brown. The number of ids in the idioplasm of the 

 pigment cells of the iris will be the same in both parents, but 

 in the case of the father nine-tenths of them, let us say, are com- 

 posed of ^ brown ^ f determinants, and only one-tenth of deter- 



* It is conceivable that individual fluctuations in the number of ids may 

 occur, although the number of idants always, indeed, remains the same — if 

 one can judge by its constancy in many animals and plants. Unfortunately 

 we do not yet know the number of idants in the case of man ; I have, at 

 least, been unable to obtain any information on this point. 



t ' Brown ' and ' blue' determinants are spoken of in this and subsequent 

 passages simply for the sake of brevity. I am not ignorant of the fact that 

 the blue colour of the iris is not due to blue pigment. The above terms 

 merely indicate that the determinant produces a structure in the iris which 

 causes it to appear blue or brown, as the case may be, quite apart from the 

 histological details on which this depends. 



