MAY, 1921. AMERICAN MARSUPIAL, C^NOLESTES OSGOOD. 115 



explanation of such variations. However, he regards "the variability of 

 a form as much a part of its specific character as any other feature of its 

 organization." Without dissenting from this, it may be added that 

 specific and especially generic characters (regardless of how they were 

 initiated or established) are themselves the result or sequence of ances- 

 tral conditions. Whether or not characters are "presence or absence" 

 characters, that is, "meristic" rather than "substantive," matters 

 little. In specialized dentitions the loss of a series of several teeth can 

 plainly be traced from species or genera in which the whole series is 

 constantly present to those in which all of it is constantly absent. In 

 such a series, the intermediate forms, or those in which the process of 

 reduction is going on, are notoriously variable. How else, it may be 

 asked, could a reduction take place, especially if discontinuous variation 

 be assumed? No better example of a variable intermediate type could 

 be had than Phalanger, which is treated by Bateson at considerable 

 length. In the artificial series from the extinct casnolestid Garzonia 

 to the modern Phascolarctos the lower antemolar teeth are reduced from 

 nine to two and the "small intermediate" teeth from seven to none. 

 Phalanger lies between the two extremes. Among 76 specimens of one 

 species (P. orientalis) examined (Bateson, p. 253), 57 had three inter- 

 mediate teeth on each side of the mandible. The remaining 19 varied 

 from those having only one on each side to one specimen having five on 

 one side and four on the other. Of these, 13 had at least three inter- 

 mediate teeth on one side of the jaw. This variability is what might 

 be expected as the result of interbreeding after the occurrence of a 

 discontinuous variation in certain individuals. Or, as seems more 

 probable to a taxonomist and comparative anatomist, it may be the 

 result of the interbreeding of individuals which have in varying degrees 

 gradually developed tendencies toward the loss of certain teeth. Such 

 tendencies are illustrated by the familiar example of the tardily devel- 

 oped and frequently absent third molar or wisdom tooth of Homo. The 

 vestigial incisors (many of which are partly calcified) which have been 

 found in all marsupials investigated by embryologists would doubtless 

 be admitted by everyone as indications of ancestral conditions. These 

 vestigial incisors are shown by Woodward and Wilson and Hill to be, 

 at least in some cases, not members of a pre-existing series (lacteals or 

 prelacteals) but persistent rudiments of suppressed members of the 

 existing series. Hence it seems not improbable that there may be cases 

 in which these vestiges arise in place and become functional as "super- 

 numerary" incisors. An embryological study of an extremely variable 

 form like Phalanger should yield interesting results bearing on this 

 point. 



