SOME CONSEQUENCES OF EVOLUTIONARY RELATIONSHIP 



383 



the outpourings of the mouselike lemming from its home in the Scandina- 

 vian uplands. Or it may be regularly -periodic, like the spring and fall 

 migrations of birds. Migration is far less likely to cause permanent exten- 

 sions of range than is spreading, though it may occasionally have this 

 result. Its causes and effects are complicated, different in different groups, 

 and only imperfectly understood. 



Lastly, plants and animals are sometimes transported accidentally, by 

 such agencies as hurricanes, floating rafts of vegetation, or mud sticking 

 to the feet of water birds. With the advent of commerce, man has become 

 the chief agent of accidental dispersal and has sometimes intentionally 



Fig. 24.19. Seed dispersal. Left, burdock, Arctium; center, milkweed, Asclepias; right, 

 dandelion, Taraxacum. (Photos by Prof. E. B. Mains.) 



but usually inadvertently carried large numbers of species to regions 

 distant from their original homes. 



Barriers and Highways. Very new species may not yet have occupied 

 all the available territory and may be seen to expand their ranges from 

 year to year. In the islands of Tahiti and Moorea in the South Pacific, 

 studies made in 1861 to 1884 and again in 1907 to 1923 showed that cer- 

 tain species of land snails of recent origin were steadily occupying more 

 territory. The same phenomenon of expansion of range is more strikingly 

 seen in the rapid dispersal of the English sparrow, the starling, and scores 

 of insect pests after their introduction into North America. 



Sooner or later, however, such an expanding population encounters 

 obstacles to its further spread. These may be physical barriers such as the 

 sea, a great river, or steep cliffs for a terrestrial species, or land for an 

 aquatic one. The ranges of few species are enclosed on all sides by phys- 

 ical barriers. Usually they are bounded along much of their periph- 

 eries by zones wherein one or more ecological factors change past the 

 limits of toleration of the species. The barrier is not a physical block to 

 entry but an inability of the species to maintain itself permanently 



