THE AMERICAN NATURALIST [Vol. XLIX 



a locus has mutated three times, each time in a different way." 

 He does not think that smaller changes than these have occurred, 

 since ' * much smaller could easily have been detected. ' ' From this 

 statement I infer that the opinion rests on casual inspection 

 rather than measurement, for which reason I do not attach much 

 importance to it. The hooded pattern of rats was not supposed 

 to vary quantitatively until its quantitative study was under- 

 taken. Two types of hooded rats were recognized, one more ex- 

 tensively pigmented than the other, and these were supposed to 

 be discontinuous like the several "mutations of a locus" in 

 Drosophila. Quantitative study has completely dispelled this 

 idea as regards the hooded pattern of rats, and I have no doubt 

 the same would be true of Drosophila. How easy it is to be sure 

 of a thing which has not yet been investigated, so sure that in- 

 vestigation of it is considered a waste of time. Muller is con- 

 fident that such variation as occurs in Drosophila " can not even 

 remotely be compared to fluctuating variability," and he gen- 

 eralizes thus : 



" In no known case do the variations of a gene among, let us say, 

 several thousand immediate descendants of the individual possessing it, 

 form a probability curve." 



The use of the word " gene " in this sweeping statement safe- 

 guards the author, since no one, so far as I know, claims ever to 

 have seen a ' ' gene " or to have measured it. How could the 

 "variations of a gene" be expected to "form a probability 

 curve " if the gene is not measurable? But if the author will 

 allow the substitution of visible character for "gene" in his 

 challenge, I will gladly accept it and I will add this generalization 

 for his consideration — No one has by actual observation and 

 measurement shown the existence of any visible character in any 

 animal which is not quantitatively variable. 



As regards the mutations of Drosophila which Muller is con- 

 fident (apparently without having studied the matter himself) 

 do not vary so as to form a probability curve, I had sufficient 

 curiosity some months ago to suggest a quantitative study by one 

 of my pupils, Mr. D. H. Wenrich. Mr. Wenrieh studied the 

 wing-length of flies from a culture kindly supplied me by Pro- 

 fessor Morgan under the name " vestigial." In advance of a 

 more detailed publication, Mr. Wenrich kindly permits me to 

 state the following facts. The wing length measured in ocular 

 micrometer units was found to vary as follows : 



