6/\. Recent Literature. [zoe 



each of the three phases of quiscula. The variations of each form 

 are then carefully followed throughout their breeding range, and 

 the general conclusions as to relationship stated. The two most 

 important conclusions are that — " In the Alleghanies of Pennsylva- 

 nia, in the Hudson Valley from Sing Sing to Troy, in eastern Long 

 Island, in Connecticut, and in Massachusetts as far north as Cam- 

 bridge, quiscula and czneus completely intergrade " ; and that — 

 " This intergradation is in every instance accomplished through 

 phase No. 3 of quiscula^ 



Mr. Chapman then argues very reasonably that qidscula is a dis- 

 tinct species, and not a race of ceneus. If this be not the case, he 

 asks why ceneus should remain so perfectly constant over an im- 

 mense area and then change into three different forms. It is, at 

 least, impossible to see any environmental influence which could 

 have produced such a modification as this, and the matter accord- 

 ingly becomes inexplicable upon any theory except hybridity. 



Although Mr. Chapman has established by his careful investiga- 

 tion at least the great probability that hybridization is the rule 

 among the grackles, he is hardly justified in extending this to other 

 species. Thus he says: " Nor do I see any good reason why we 

 should refuse to admit hybridization as a factor in the evolution of 

 what we term species. * * * Difference in habit under what 

 must necessarily be similar conditions will ever be an effectual bar- 

 rier against the indiscriminate mixing of even closely - allied birds. 

 But when two species whose natural economy, song, nidification, 

 etc., are the same, and which agree in structural details and differ 

 only in coloration, inhabit contiguous regions, is it unnatural that 

 they should at first occasionally, and in the end regularly, inter- 

 breed ? The evidence in proof of such intergradation is gradually 

 accumulating, and in the future I think we shall be forced to recog- 

 nize hybridization, not only as a means which unites known forms, 

 but which also gives rise to new ones." 



The writer has apparently overlooked, in the above passage, the 

 possibility of physiological selection interposing a barrier to hybrid- 

 ization, even when the two species appear to be structurally iden- 

 tical. If the theory of physiological selection is to have any valid- 

 ity whatsoever, it is necessary to assume that such cases of habitual 

 hybridization as are occasionally recorded, are exceptional and ab- 

 normal. To be sure, it may be objected that this is arguing from 



