^•o. 490] MUTATIONS AND DISTRIBUTION 655 



words, intergrading forms of a common stock. It is therefore 

 perfectly evident that botanists and zoologists are often speaking 

 of entirely different concepts when discussing the occurrence or 

 non-occurrence of species in the same area. It is also evident that 

 minor forms among plants bear no relation to the niiiior forms 

 among animals, either in mode of origin or in inaimci- of distribu- 

 tion. In Crataegus, Rubus, Amelanchicr, \ iohi, Aster, and 

 countless other generic groups of plants, there often occur many 

 slightly differentiated forms growing side by side over large dis- 

 tricts. Among animals, at least among vertebrates, no such con- 

 ditions appear to obtain; the slightly differentiated forms occupy 

 different areas, and \vh(>re the borders of their breeding ranges 

 approach tiiey uradually ni(>rge tlie one into the other with tiie 

 gradual change in the environment. In the case of tlie |)huits 

 mentioned, these slight differentiations niiiintain themselves de- 

 spite similarity of environment; in the case of the ahiinals, they 

 are obviously the product of environment. The oiiuiii .'f such 

 plant forms may never be discovered, but to many mimis their 

 development by mutation may seem not iinj)n»bable. >i> long as 

 we do not find similar conditions among the liii^lier animals, it 

 is hard to see how mutation has been active u\ oriixinaiion of 

 new forms, whether s{)eeies in the nsnally a( ( cptcMl sen.se, or the 

 minor variants usually reeounized as ii!( i|)i('nt si u'cics or subspecies. 

 With these facts and conditions in vieu I )v. Leavitt's alwve- 

 ■quoted suggestion that "zoologists may best discover the condi- 

 tion and interpret its meaning among animals, and botanists among 

 plants," is eminently worthy of serious consideration. It is "ob- 

 viously unsafe," as he well says, to reason deductively from one 

 kingdom to the other. 



and zoologists on the snbjecls of '"nnitation" and tlie "distribution 



impression that much of the oi)]u)sitioii of views <»n tlies,- (|ut>stions 

 is due in part to too s\v(>e|)ini:- assertions by Itoili botanists and 

 zoologi.sts, in part to a n.isnnderstanding by one side of what the 

 other side really means, and lari^elv to dcdnctivt- i-easoning from 



