CAIN: CENTER OF ORIGIN 133 



The literature of plant and animal geography, taxonomy, and evolution is 

 replete with statements concerning the center of origin of certain species, spe- 

 cies groups, genera, etc. For example, Babcock and Stebbins (1937) say, "The 

 distribution of the genus Youngia taken as a whole is entirely consistent with 

 the conception that it is a natural group which had its origin in southeastern 

 Asia and that evolution has been accompanied by extension of the geographic 

 range." On the other hand, some species, as recognized by taxonomists, may 

 not have had a center of origin in the sense of a restricted geographic spot 

 where they arose. For example, Gleason (1923) states: 



"... Probably if a complete series of specimens were at hand, showing 

 comprehensively the maples of the eastern states, for example, from the Plio- 

 cene to the present time, it would be seen that some of the earlier forms are 

 absolutely continuous with our present species and that the slight morphological 

 distinctions between them are only the result of continuous slow variations 

 throughout the centuries. According to this view, many modern species had no 

 localized origin and are not the off shoot of any parent, but represent the mass 

 development of a species, which, under our present taxonomic ideas, came to a 

 stop at the beginning of a break in our geological record of it and reappeared 

 as a new species at the beginning of our next experience with it." 



A different situation is emphasized by Kinsey (1936), who denies both the 

 usefulness and the truth of the concept of center of origin. He demonstrates 

 through his taxonomic work with the gall wasps that species differ by many 

 genie factors that have been added gradually to the population as it has mi- 

 grated. Some of the characters of a species have been added in one place, and 

 others in other places, and certain gene frequencies have increased with isola- 

 tion resulting from migration. Where, then, is the center of origin? I think it 

 would be begging the question to say that the center of origin of a species is 

 where the genie factor or factors causing reproductive isolation arose. 



Two other situations can be mentioned in which, in the strictest sense, there 

 is no single center of origin.' Chromosome (genom) doubling may happen 

 many times in many places in a diploid population. The resulting autotetra- 

 ploids, which may be a good species, do not necessarily have a center of origin 

 other than the area of the entire progenitor diploid population. The map of 

 Baldwin (1941) showing the chromosome races at Galax aphylla is of interest 

 in this connection. Also, it is becoming increasingly apparent that many plant 

 species are of hybrid origin. Sometimes a swarm of diploid hybrids, segregates, 

 and backcrosses have attained a sufficiently distinct character and area that their 

 population has been given specific status, distinct from the original species. At 

 other times polyploid complexes develop. Stebbins (1940) says, "Dissolution 

 of genetic barriers and exchange of genes between genetic systems that are 

 completely isolated from each other in the diploid condition are made possible 

 by the synthesis of polyploid complexes through allopolyploidy between three, 



