6 BULLETIN 61, UNITED STATES NATIONAL MUSEUM. 



as they always have been in this genus. And again while the law that 

 the direct relative of any form will generally be found in a neighboring 

 environment (see p. 192) may not be universal among the vertebrates 

 the evidence at the present time shows that it is valid in a great 

 number of cases, so that one must be very cautious of this point in 

 grouping the forms of any group. 



Cope in his general work recognized five varieties of sirtalis east of 

 the Mississippi River, and yet the range of these forms could not be 

 defined relative to each other, as they were reported from various 

 parts of the same region and in association with numerous other 

 varieties of the same stock, in utter defiance of the above law, but as 

 most of these forms were based on slight variations and have been 

 mostly dropped, they do not merit detailed discussion. The aflinities 

 that have been claimed for recognizedly distinct forms, however, is 

 open to the same criticism. For example, if ordinoides and concinnus 

 are both derived from pariefalis, one must invoke the aid of some form 

 of phj^'sio logical isolation to account for their present distinctness, for 

 they occupy the same region and environment, and concinnus is not 

 considered far enough separated from the parent stock to be classed 

 as a distinct species. Similarly also the derivation of radix from 

 sirtalis is different from what one might expect from their geographic 

 location , for radix lies entirely within the range of sirtalis and parietalis. 

 Thus geographic probability must also be observed in tracing genetic 

 lines if we would avoid the grouping of forms of diverse origin , which 

 by their similarity might reasonably be adjudged to be related. 



Having determined the significance of the variations, and by 

 observing the geographic probabilities, considerable advance may be 

 made toward determining genetic relationships, and it only remains to 

 look for similarities and intergradations. For example, we may thus 

 have determined that the smaller scutellation of ordinoides may have 

 been derived by dwarfing from some other form with a larger number 

 of scales, and that it is possible, geographically, that it may have been 

 derived from the snakes of southern California or from those in the 

 Great Basin. It is not enough to say that the present combination of 

 traits which we call ordinoides is constant, but we must cast about for 

 similarities with neighboring forms in order to determine its origin if 

 we are to avoid the multitudinous varieties of systematists whose 

 vision is blurred by the present divergences. The species-describer 

 searches for pure types which are found only away from the geo- 

 graphical meeting places, but if we wish to discover relationships we 

 must not avoid the boundary lines, but seek them. If we search for 

 similarities in the light of the influence of modifying factors on the 

 characters, and with an eye to geographic probabilities, we should be 

 able to see back of the present diversities, grasp the natural affinities, 

 and come in touch with the questions of origin and descent. It was 



