146 MENDELISM chap. 



should occur in the one sex, but not in the other, 

 is at present unexplained. 



If it were found by direct observation that the 

 behaviour of the chromosomes differed in the tvi^o 

 sexes, we should be furnished with substantial evidence 

 in support of the chromosome theory. But at present 

 we are without grounds for this assumption. Indeed 

 it should not be forgotten that the postulated twist- 

 ing of the chromosomes on one another, though 

 observed in some animals, has never yet been de- 

 monstrated for the female of Drosophila. The 

 technical difficulties are doubtless very great, yet 

 until this demonstration has been provided there 

 must necessarily remain a gap in the chain of evidence 

 by which the chromosome theory is supported. 



To attempt to follow the theory in all its impli- 

 cations would lead us far beyond the scope of this 

 little book. There are many difficulties in the way 

 which must be cleared up before it can be accepted 

 without reserve. So far it is based entirely upon 

 the Drosophila work, in which it is claimed that 

 the number of the groups of factors exhibiting the 

 relation of linkage is equal to that of the number 

 of chromosomes in the gamete. If it were discovere4 

 that in any animal or plant there existed a number 

 of factors, or groups of factors, without linkage 

 relations, greater than the gametic number of 

 chromosomes, the theory would necessarily fall to 

 the ground. No genetic material hitherto used 

 gives such quick returns as Drosophila, and probably 

 some years must elapse before we can hope for 

 definite evidence from other forms of life. Until 

 this is forthcoming it is perhaps as well to exercise 



