172 THE AMERICAN NATURALIST [Vol. LVI 



differences that there may be are probably qualitative 

 rather tlian quantitative. It is possible that there may be 

 very little difference in the amount of essential hereditary 

 material. But the word " amount " must be considered 

 as equivalent portions. The visible size of the chro- 

 matin mass fluctuates greatly even at different stages of 

 growth in the same individual. 



Although the cytological proof of the chromosome 

 theory is still so meager as to make speculation somewhat 

 useless, nevertheless, looking at the matter from the stand- 

 point of difficulty of recombination the important consid- 

 eration is the number of fifty-unit lengths of chromo- 

 somes. However, the Morgan school unit of measure- 

 ment, the one per cent, of crossing over, is not a stable 

 unit, as they have shown that crossing over fluctuates 

 rather disconcertingly, due l)oth to environmental and ger 

 minal modifying factors. Detlef sen' finds the rate of cross- 

 ing over between certain loci to be very profoundly altered 

 by continued selection for high and low cross over stock. 

 So that for the present the terms proposed by Haldane* 

 of morgan and centimorgan as measures of chromosome 

 length do not have any precise application. At the same 

 time the rate of crossing over is the only measure avail- 

 able and can not be given up until a better one is found. 

 The term morgan, referring to a one-hundred-unit length 

 of chromosome is convenient but does not have the bio- 

 logical significance that a fifty-unit length of chromosome 

 would have. Since every gene is independent in trans- 

 mission from all other loci in the same chromosome more 

 than fifty units distance from it, and has the usual Men- 

 delian relation with them as well as with all the factors in 

 the other chromosomes, the term mendel would perhaps 

 be useful, if the employment of such terms can be justified 

 at all. Applied in this way a mendel is a measure of 

 chromosome length equivalent to fifty per cent, of cross- 

 ing over. 



It should be noted that a mendel is not comparable to a 



1 Proceedings of the National Academy of Science, 6: 663-670, 1920. 



2 Journal of Genetics, 8: 299-309, 1919. 



