Xo. 570] 



SPECIES-BUILDING 



337 



est to determine whether such true-breeding white 

 females actually occur in California. 



Evolution in Colias is usually regarded, on the other 

 hand, as tending towards suppression of the white stock 

 rather than its further extension, inasmuch as Pieris and 

 other allied genera are white. It seems to be a reason- 

 able hypothesis that, by progressive mutations in Colias 

 affecting first the male then the female, 18 white has be- 

 come yellow; yellow, orange; orange, red, or a fiery 

 orange; 19 or yellow may be transmuted into black, as in 

 an aberration of the male in C. philodice. By retrogres- 

 sive or degressive mutations, accordingly, we may hope 

 to isolate from C. philodice or C. eurytheme el pure white 

 race. 



SUMMABY AND CONCLUSIONS 



The erroneous idea that Linnsean species are homo- 

 geneous, well-defined groups of equal importance has 

 done much to retard progress in the experimental study 

 of evolution. The limits of a species are often arbitrary, 

 depending ultimately upon the temperament of the des- 

 criber, and frequently based upon ignorance of the near- 

 est allies of the individuals described, living in other 

 parts of the world. 



The most definite criteria of species, viz., that " spe- 

 cific" characters are constant, and that hybrids of 

 Linnaean species are infertile inter se, are only approxi- 

 mately correct. Characteristics of species sometimes 

 occur in heterozygous condition. Hybrids of Linnaean 

 species, as has long been known, are often fertile. These 

 matters, owing to traditional, unwarranted respect for 

 described species, have received comparatively little in- 

 vestigation. 



Examples of hybridization in Adalia, Colias, Meganos- 

 toma, Basilarchia and Paratettix among insects, in Co- 

 laptes, Quiscalus, and Helminthophila among birds are 

 cited. 



