ISOLATING MECHANISMS AND SPECIES FORMATION 



lands. The flora and fauna of mountains are similarly restricted in their 

 dispersal by their inability to cross the lowlands intervening between 

 neighboring mountains or mountain ranges. Mayr studied the mountain 

 birds of New Guinea, and found that almost all of them have broken up 

 into subspecies in much the same fashion as have the birds of archi- 

 pelagos. In some cases, there are even series of distinct altitudinal races, 

 including lowland, mid-mountain, and alpine races. Many mammals such 

 as the mountain goat and bighorn sheep are limited to the highest moun- 

 tain habitats. Turel found that a very high proportion of alpine plants 

 belonged to endemic races. 



Similarly mountains will serve as barriers to lowland organisms. The 

 American opossum, Didelphis virginiona, ranges widely in the eastern 

 United States. The low eastern mountains do not comprise a barrier to it. 

 It was introduced into California nearly 90 years ago for sporting pur- 

 poses. It has thrived in the low coastal range, but has been unable to 

 invade the Sierras. The thirteen-lined ground squirrel, Citellus tridecem- 

 lineatus, is widely distributed in the prairies of north central United 

 States, but it stops short of the Rockies. The cottontail rabbit, Sylvilagus 

 fioridanus ranges over most of the United States east of the Rockies, but 

 has been replaced in the mountains by its cousin, the jack rabbit. The 

 white-footed mouse, Peromijsciis leucopus, is similarly distributed, while 

 its near relative, P. maniculatiis, has successfully invaded the mountains. 



Similarly, extensive forests will serve as barriers to dispersal of grass- 

 land organisms, prairies will serve as barriers to forest organisms, and 

 even finer classifications are practical. Thus, the red tree mouse, Phena- 

 comijs longicaiidus, lives on a diet of fir needles. It also nests in fir trees, 

 and spends most of its life in them. Thus, not only will prairies serve as 

 a barrier to its distribution, but non-fir forests will be very effective bar- 

 riers. Even simple distance may serve as a barrier, for Dice and Blossom 

 found that seven species of small mammals were subspecifically distinct 

 at Tucson and at Yuma, yet there seemed to be no barrier to their free 

 dispersal other than distance. As stated at the beginning of this discussion, 

 there is scarcely any natural feature which may not be a barrier to some 

 plant or animal. And the barrier of one is the highway of dispersal of 

 another. 



Thus far, only geographic factors in the restriction of random dispersal 

 have been mentioned. But Mayr has included several characteristics of 

 animals which tend to limit their dispersal even in the absence of geo- 

 graphic barriers. One of these is the sedentary character of many animals. 

 Surprisingly enough, it is not sessile animals, like the sea anemones, to 

 which this applies with greatest force, for the small, free-swimming larvae 

 of these marine invertebrates may be widely distributed by ocean cur- 

 rents. Similarly, most plants, although strictly sessile, have means of dis- 

 persal of seeds which are at least as efficient as typical means of dispersal 

 for animals, and commonly much more so. But the larger animals, par- 

 ticularly the vertebrates, whose locomotor organs are among their most 

 obvious characteristics, seem to use their powers of locomotion to main- 

 tain their home range far more than for range expansion. The very exten- 



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