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THE AMEBIC AX XATURALIST [Vol. XLIII 



As already noted, while the group is a compact one, through 

 the close interrelation of all the forms referred to it, the ex- 

 tremes of differentiation are widely diverse, but there are no 

 trenchant, lines of division, for, "if a single character becomes 

 pronounced, it is merely an extreme development which may be 

 traced back by stages to a widely different condition. ' ' Mr. Os- 

 good, however, considers it desirable to recognize six divisions as 

 subgenera, in order to indicate clearly the relationships of the 

 diverse forms included in the genus. Two of them (Podomys 

 and Ochrotomys) are here for the first time characterized; two 

 others (Baiomys and Megadontomys ) have been accorded by 

 some authors the rank of genera, mainly as a matter of conveni- 

 ence in dealing with so large a group. 



It would require too much space to go into details respecting 

 Mr. Osgood's methods of dealing with the specially difficult cases, 

 but the general outcome of his researches is of such general inter- 

 est and has such a wide bearing that it seems desirable to give a 

 few passages from his general introduction in his own words. 

 Thus, under the heading "Variation" (p. 16), he says: 



sporadic nature often occur Ilia! one may almost believe them to have 

 been produced in one or at most a very tew generations. Such varia- 

 tions, of course, are slight, and doubtless produced immediately upon 

 contact with certain conditions. Thus if the range of a given form 



an appreciably darker color than the normal form occupying the sur- 

 rounding region. And whenever similar conditions are repeated else- 

 where, even on a small scale, the same result seems to follow. Again, 



In the absence of absolute proof one can scarcely avoid tl e s sp o 

 that if the progeny of paler individuals were transferred at an early 

 age to the habitat of darker ones, thev would, quite regardless of in- 

 herent tendencies, develop a darker color, or, similarly, a lighter color 

 if the process were reversed. 



