THE DETERMINATION OF DOMINANCE. 335 



although the organisms may be dose together, living in the same 

 location, etc., by definite limitations in their reproductive ac- 

 tivities prevents them from intercrossing. This may be due 

 to incompatibility between germ cells, but quite often it is due 

 to difficulties incident to copulation or to the penetration of 

 sperms; that is, the difficulties are purely mechanical. Thus it 

 might well rarely happen that an individual would arise of a char- 

 acter such that a cross could result; and if such a cross were 

 made, a race having the attributes of one organism with the 

 capacity for physiological isolation of the other, might easily 

 arise and keep it from any further chance of intercrossing with 

 other species. 



Conclusion. 



The experiments and observations herein given warrant the 

 general statement that conditions external to a cross are impor- 

 tant factors in determining the results thereof. This conclusion 

 has been worked out in both normal and hybrid crosses, in 

 crosses between races which have been created selectively, and 

 between forms which arose as sports; and the second series of 

 experiments in synthesis is sufficient warrant for attributing to 

 this factor a considerable importance in evolution. 



Underlying these, there are, of course, deeper factors than 

 those with which we are dealing. The characters which behave 

 Mendelianwise are in the main superficial, unimportant attributes 

 of the organism, and only rarely are they the characters which 

 would make for success or failure in the struggle for existence; 

 they are most often color and specific characters, which, while 

 fixed and vigorous in their behavior, are not important in the 

 economy of the organism. These behaviors, beyond any ques- 

 tion as to how and why, suggest the operation of something 

 which gives a result best described at present in factorial 

 terms ; and that there are such things as later ontogenetic factors 

 seems highly probable in many cases and absolutely certain 

 as regards many colors. The knowledge that we have concern- 

 ing melanogenesis leaves us no alternative in this respect. 



Back of all this, however, is the fundamental question of how 

 these germ cells, how this living substance is constituted, and what 



