712 



THE AMERICAN NATURALIST 



[Vol. LI 



to appear in the individual development. They for the 

 most part affect the size and shape of the wings, the size, 

 shape and color of the eyes, and the color of the body. 



If a systematist were asked whether these new races of Drosophila 



He would call them all o^e species. If he were asked why, he would 

 say, I think, "These races diifer only in one or two striking points, 

 while in a hundred other respects they are identical even to the minutest 

 details." He would add, that as large a group of wild species of flies 



in nearly every detail and be identical in only a few i)()ints.-''^ 



This point of view seems justified, since the foremost 

 dipterologist in this country, a man who has named over 

 one thousand species and genera, mostly of flies, says 

 regarding the results of certain experiments carried on 

 with Drosophila by one of his colleagues.^^ 



But I think it is absolutely certain — and I speak as an entomologist 

 fairly familiar with flies — that it would be impossible to produce species 

 of his sports even though they were bred for a thousand years.^^ 



In talking over this species question with one who has. 

 had considerable experience in systematic work^*^ it be- 

 came clear that although as a rule only a few of the more 

 coTisiiiciioiisly contrasting characters are selected for de- 

 >riii,ti\,. i)iii i)oses, as a matter of fact the individuals of 

 (lifffi^Mit -jM'cies are often different in practically every 

 morphological characteristic. One who is very familiar 

 with these species will realize these differences at once, 

 although many of them are of such a nature that they 

 can not be described so that any one else will recognize 

 them. There seems to be no difficulty, however, in finding 

 numerous describable contrasting characters in Droso- 

 phlhi, since at least fifty-nine are included in the descrip- 

 tions of two recently named s]iecies''i that were selected 



37 Morgan, 1916, "Critique of the Theory of Evolution." 



38 Dr. F. E. Lutz. 



39 WUliston, 1908, Amer. Nat., Vol. 42. 



