THE AMERICAN NATURALIST [Vol. XLVIII 



distinct species. Arbitrary rules for species making, de- 

 signed to restrict the activities of the more vigorous 

 " splitters " have been indeed laid down by experienced 

 and conservative systematists. The final test, however, 

 so far as any exists, is acknowledged to be whether a 

 group breeds approximately true to its kind and is ap- 

 proximately sterile with other closely related stock, and 

 yet in how few cases have both or either of these criteria 

 been actually applied by the describer of species ! 



As a matter of fact no stock that has been bred on a 

 vast scale, so far as I am aware, breeds absolutely true to 

 specific characters. In Morgan's Drosophila 1 and De 

 Vries's (Enothera, numerous mutants appear, probably 

 through the absence of certain chemical elements, or by 

 unusual combinations of elements, in the chromatin of the 

 germ plasm. That this phenomenon has not been shown 

 for many other species is due, in all probability, to lack 

 of close attention to all the individuals in a huge proces- 

 sion of stock in the process of breeding. Any insect bred 

 as extensively as Drosophila ampelophila, the pomace fly, 

 has been would probably show as many mutants; some 

 would show more. Colias eury theme, the " orange sul- 

 phur " or alfalfa butterfly, is such an example. Though 

 this butterfly can not be bred on a scale comparable with 

 Drosophila, every thousand individuals yield many dis- 

 continuous variations : red eyes instead of green, tongue 

 uncoiled instead of wound in close flat spiral when at rest, 

 one antenna shorter than the other, the absence of certain 

 spots from the wings, gynandromorphism, caterpillars 

 with two longitudinal rows of large black dorso-lateral 

 spots or white dorso-lateral stripes upon a dorsal surface 

 usually unmarked, caterpillars with one proleg less upon 

 one side than the other. This is a partial list of points at 

 which the descendants of three females of Colias eury- 

 theme failed in a single summer to breed true to the char- 

 acteristics of the species, though bred under uniform 

 normal conditions. The fact that these discontinuous 



» Science, N. S., Vol. XXXIII, Nos. 847, 849, pp. 496-499, 534-537, 1911. 



