THE BlULOCilCAL CONQUEST OF THE WOULD n 



Human Interference 



Man is often the unwitting cau.se of sliifts, .sometimes with serious 

 results, of animal and ])lant ixjpulations. Tlie Russian thistle, 

 already mentioned, was introdueed into South Dakota in 1S74 with 

 flax-seed from Europe. By 1888, it was reported as a troublesome 

 weed in both the Dakotas. By 1898, it had covered all the area east 

 of the Rocky Mountains from the Gulf of Saskatchewan, and today 

 ranges over the whole country. 



There are many curious cases of the accidental transport by human 

 agency of animals and plants to regions far from their point of origin. 

 Recently a tropical boa landed in Middletown. Connecticut, with a 

 bunch of bananas. Tropical tarantulas, too, are known to be carried 

 over long distances in the shipment of this fruit. Such instances as 

 these, however, usually have no lasting effect on the general spread 

 of organisms, yet they emphasize the fact that unanticipated develop- 

 ments in distribution are quite jiossible from very insignificant and 

 unsuspected beginnings. Man's interferences with the distribution 

 of organisms have by no means always been unfortunate or disastrous. 

 In many instances his rearrangements of plant and animal popula- 

 tions have been eminently successful. The planting of various 

 species of trout in new streams has proved to be a wise move, \\hile 

 the introduction of reindeer into Alaska and Labrador is of incal- 

 culable benefit to both man and beast. The list of cases where man 

 has lifted the lid of Pandora's box and set free plants and animals 

 for weal or woe into new localities could be extended indefinitely. 



Life Zones 



Reference has already been made to a zonal distribution of i)hints 

 and animals in a pond. A similar condition is easily seen in climbing 

 any high mountain. Life zones are often rather sharply marked, but 

 usually show transitional areas between them. A region which has 

 been carefully studied and which shows this zonal distribution in a 

 marked way is the San Francisco mountain region in north Arizona. 

 Here, a mountain nearly 13,000 feet in height rises out of a desert 

 plain. This mountain shows successively two tyj^es of desert zone, 

 a lower and upper, each with its own desert fauna and flora, cacti, 

 sagebrush, a few birds, mice, lizards, and snakes. Then a r(>gion at 

 between 6000 and 7000 feet of pinon pines and red cedars, inhabited 

 by more birds and a small number of mammals. Between 7000 and 



