418 



THE AME 



ITURAL1ST [Vol. XLIII 



haps be demonstrated. It is the general experience that 

 blueness is dominant over its absence, but this is just the 

 result I have pictured here as a case in which absence of 

 redness or of acidity is dominant over its presence. I 

 know of no way of determining whether red flowers are 

 blue flowers with an added factor for acidity, or whether 

 blue flowers are red with an added factor for alkalinity, 

 and, indeed, it is conceivable that both of these situations 

 may be presented in different species. However, my 

 purpose . is attained if I have shown that there is no 

 greater theoretical difficulty involved in the dominance of 

 absence over presence than in the dominance of presence 

 over absence, and that the assumption that any given 

 character is due to the presence of an added internal unit 

 does not "imply the dominance" of that character. 



The non-appearance of an externally visible character 

 in the heterozygote, although the corresponding internal 

 unit is present, as must always be the case when real ab- 

 sence is dominant over presence, plainly presents a kind 

 of latency somewhat different from the four types recog- 

 nized by me 13 in a recent article in the American Nat- 

 uralist. For the sake of uniformity with the termin- 

 ology there adopted I may call this new kind of invisibil- 

 ity "latency due to heterozygosis." Like all the other 

 types of latency except that due to fluctuation, the latency 

 resulting from heterozygosis produces no deviation from 

 definite characteristic ratios. 



I recall at present only one case in which we can cer- 

 tainly identify latency due to heterozygosis, for the 

 reason that, just as we have seen above in regard to blue 

 and red flowers, it may be quite impossible in any partic- 

 ular instance to decide which is the positive character and 

 which its absence. In a particularly interesting cross 

 between a yellow and a reddish snail, Lang 14 has found 



Latency," American Naturalist, 42: 433-451, July, 1908. 



"Lang, A. TJeber die Bastarde von Helix hortensis Miiller und Helix 



