GEOGRAPHIC DISTRIBUTION 311 



as the other, and it is fairly certain that the one with the larger number 

 sprang from the other by a doubling of the chromosomes. This is a 

 weh known method of origin of species in plants, and must apply to this 

 example. However, the species with the double number of chromosomes 

 (which must be the younger one) ranges much more widely than the one 

 with the smaller number. One species is simply much more successful 

 than the other. 



Continuity of Range. — Because a group of animals starts at some 

 point, from which its members tend to spread until barriers are reached 

 on all sides, ranges are expected to be continuous unless something 

 happens to break them up. Taxonomic groups of as high rank as 

 families and orders have usually been developing long enough for that 

 "something" to take place. Ranges of such groups are large, and living 

 conditions may change sufficiently to extinguish the animals across the 

 middle of the area, thus dividing it in two. The camel family, for 

 example, is represented by the true camels in Asia and Africa, and by 

 the llama and its relatives in South America, wdth the great land gap 

 of Europe and North America between. Fossil camels, however, are 

 found in the area now vacant. The genus Alligator is composed of two 

 species, one in central China, the other in southeastern United States. 

 Extinct relatives of the alligators once ranged widely in North America 

 and Europe, shoAnng how the modern range became discontinuous. 



When the range of a species is found to be discontinuous, which is 

 rare, the reason is not easily found. The skink, Lciolopisma laterale, is 

 found in southeastern United States, in China, and in certain of the 

 southern Japanese islands. Why is it not in the areas between? Only 

 if the species is an exceedingly old one would it be hkely that destruc- 

 tion of its members over a large portion of its former range could have 

 occurred. So improbable is the division of a species range by extinction 

 that every example of it raises the question whether the species may not 

 have developed independently in two places. Such an occurrence is not 

 impossible. j\Iany mutations are known to be produced repeatedly, and 

 among random recombinations of genes the same combinations could 

 occur anywhere. If environment of a certain type tends to preserve 

 certain genetic combinations, similar environments in two areas could 

 guide evolution in the same direction. Such double origin of a species 

 would not be a violation of the taxonomic concept that all members of a 

 species are descendants of common ancestors, for the two groups from 

 which the species arose would necessarily be much alike, both having 

 come from the same ancestry. The common ancestry of a duplicated 

 species would thus be simply pushed farther back. Nevertheless, this 

 dual origin of a species is so unlikely that it is not to be lightly assumed 

 as an explanation of discontinuity. 



