CAIN : CENTER OF ORIGIN 147 



Let us turn again to the often cited polyploids. Anderson (1937) says, 

 "The diploid species are of limited distribution and even in those areas where 

 they do occur are usually restricted to one particular habitat. By contrast, the 

 tetraploid species and races have wide distributions and most of them have 

 the ability to flourish under a variety of situations." Allopolyploids, especially, 

 may combine the tolerances of their diploid progenitors. 



In amplifying his discussion of this criterion, Adams (1909) selects what 

 seems to me to be a particularly vulnerable example. He says, "Outlying colo- 

 nies tend to have a limited or restricted range. At the same time such colonies 

 are peculiarly liable to become extinct, as they are usually near the limit of 

 favorable conditions . . . this is true of the 'boreal islands' in swamps within 

 the glaciated portion of the continent. For example, members of the tamarack 

 bog association, toward their southern limit, have very restricted or local range ; 

 but to the north, the bog forest conditions, as it were, spread from the bogs 

 proper and become of extensive geographic range, as the water beetles invade 

 the damp mosses . . . These restricted, attenuated, or isolated colonies, depend- 

 ent upon special conditions, are clearly indicative that they are pioneers or 

 relics, which point toward the region where the range is spread out and becomes 

 of geographic extent." I have italicized a portion of the above quotation to em- 

 phasize the fact that the areal pattern is apparently wholly dependent upon the 

 pattern of occurrence of suitable conditions. This is an ecological matter that of 

 itself denotes nothing concerning origin. Adams goes on to say that the isolated 

 colonies are either pioneer or relic, destroying his own thesis, it seems to me. 



Criterion 8. Continuity and Directness of Individual Variations or 

 Modifications Radiating from the Center of Origin along Highways 



OF Dispersal 



This criterion, related to number six, frequently is a reliable one. With 

 respect to changes in character frequency (as shown by the mass-collection 

 techniques : Fassett, 1941) we can only conclude that there can be a gene flow 

 in any direction through a population. Any attenuation of the frequency of a 

 certain gene is presumably direct evidence of the center of origin of that gene 

 in the region of highest frequency. One of the most interesting cases of this 

 sort concerns the distribution of the recessive melanistic mutation in Cricetus 

 cricetus, the hamster. Timofeefli-Ressovsky (1940) says, "In the course of the 

 last 150 years this mutation has spread from its original center of high concen- 

 tration along the northern border of the species-area . . . populations with 

 rather high concentration of this gene are spread westward as far as the river 

 Dnieper." Apparently the melanistic form is adaptive in the wood-steppe eco- 

 tone along the northern portion of the species area, and this is one of the few 



