August 28, 1914] 



SCIENCE 



297 



fess to disting-uish specific limits and to 

 declare that this is a species and that a 

 variety. The only definable unit in classi- 

 fication is the homozygous form which 

 breeds true. When we presume to say that 

 such and such differences are trivial and 

 such others valid, we are commonly em- 

 barking on a course for which there is no 

 physiological warrant. Who could have 

 foreseen that the apple and the pear — so 

 like each other that their botanical differ- 

 ences are evasive — could not be crossed to- 

 gether, though species of antirrhinum so 

 totally unlike each other as majus and niolle 

 can be hybridized, as Baur has shown, with- 

 out a sign of impaired fertility? Jordan 

 was perfectly right. The true-breeding 

 forms which he distinguished in such multi- 

 tudes are real entities, though the great 

 systematists, dispensing with such labori- 

 ous analysis, have pooled them into arbi- 

 trary Linnean species, for the convenience 

 of collectors and for the simplification of 

 catalogues. Such pragmatical considera- 

 tions may mean much in the museum, but 

 with them the student of the physiology of 

 variation has nothing to do. These "little 

 species," finely cut, true-breeding, and in- 

 numerable mongrels between them, are 

 what he finds when he examines any so- 

 called variable type. On analysis the 

 semblance of variability disappears, and 

 the illusion is shown to be due to segrega- 

 tion and recombination of series of factors 

 on predetermined lines. As soon as the 

 "little species" are separated out they 

 are found to be fixed. In face of such a 

 result we may well ask with Lotsy, is there 

 such a thing as spontaneous variation any- 

 where ? His answer is that there is not. 



Abandoning the attempt to show that 

 positive factors can be added to the original 

 stock, we have further to confess that we 

 can not often actually prove variation by 

 loss of factor to be a real phenomenon. 



Lotsy doubts whether even this phenom- 

 enon occurs. The sole source of variation, 

 in his view, is crossing. But here I think 

 he is on unsafe ground. When a well- 

 established variety like "Crimson King" 

 primula, bred by Messrs. Sutton in thou- 

 sands of individuals, gives off, as it did a 

 few years since, a salmon-colored variety, 

 "Coral King," we might claim this as a 

 genuine example of variation by loss. The 

 new variety is a simple recessive. It differs 

 from "Crimson King" only in one respect, 

 the loss of a single color-factor, and, of 

 course, bred true from its origin. To account 

 for the appearance of such a new form by 

 any process of crossing is exceedingly diffi- 

 cult. From the nature of the case there can 

 have been no cross since "Crimson King" 

 was established, and hence the salmon must 

 have been concealed as a recessive from the 

 first origin of that variety, even when it 

 was represented by very few individuals, 

 probably only by a single one. Surely, if 

 any of these had been heterozygous for 

 salmon this recessive could hardly have 

 failed to appear during the process of self- 

 fertilization by which the stock would be 

 multiplied, even though that selfing may 

 not have been strictly carried out. Exam- 

 ples like this seem to me practically con- 

 clusive.^ They can be challenged, but not, 

 I think, successfully. Then again in re- 

 gard to those variations in number and 

 division of parts which we call meristic, 

 the reference of these to original cross- 

 breeding is surely barred by the circum- 

 stances in which they often occur. There 

 remain also the rare examples mentioned 

 already in which a single wild origin may 

 with much confidence be assumed. In spite 

 of repeated trials, no one has yet succeeded 

 in crossing the sweet pea with any other 

 « The numerous and most interesting ' ' muta- 

 tions" recorded by Professor T. H. Morgan and 

 his colleagues in tlie fly, Drosophila, may also be 

 cited as unexceptionable eases. 



