ZOOGEOGRAPHY 303 



is furnished by the potato beetle, Leptinotarsa decemlineata, whose mi- 

 gration has been briefly summarized by Tower as follows: 



"The original distribution of decemlineata was on the eastern slope of the Rocky 

 Mountains northward to the Canadian boundary, eastward into western Kansas 

 and Nebraska, and southward into Texas and New Mexico. In this habitat 

 it was found by Say in 1823. Then, as now, it was probably sparsely distributed 

 over the area, feeding upon Solanum rostratum. I know of no record of its having 

 been found at an elevation over 8000 feet, or in the Great Basin. 



"In this habitat it remained in stability until 1845 or 1850, when the western 

 extension of human colonization introduced into its habitat a new factor by the 

 addition to the flora of a new plant which proved to be an acceptable food, Sol- 

 anum tuberosum. 



"About 1845 or 1850 the settlers in the Mississippi valley, in making their ad- 

 vance westward, brought the cultivated potato into the edge of the habitat of 

 decemlineata, where the beetle soon learned tc use it as a food. This extension 

 of the area wherein S. tuberosum was grown into the habitat of decemlineata 

 resulted in the removal of a previously existing barrier to further eastward ex- 

 tension. This barrier was the wide stretch of country in which no food plant 

 had hitherto been available and into which it could not go without food. The 

 advent of this new food, however, completely removed the barrier and there lay 

 open to the eastward an expanse of territory where optimum conditions of exist- 

 ence were developed. Into this area it began to spread — at first slowly and in 

 small numbers and unnoticed; then in increasing numbers and more rapidly, 

 making itself felt as an economic factor in agriculture. Onward it advanced 

 yearly, in increasing numbers and speed, until twenty years from its start it had 

 reached the Atlantic coast, a barrier beyond which it has not been able to suc- 

 cessfully pass. Fortunately the history of this advance is known from extensive 

 records, so that it can be traced in detail from year to year." 



Although the phenomenon of normal migration is most easily seen 

 and comprehended in active terrestrial species it is an attribute of all 

 forms whether free-moving or fixed. In fixed aquatic forms and in 

 some slow-moving species the young are hberated as free-swimming 

 organisms, or they become attached to free-swimming animals and thus 

 bring about an increase in the range of the form. For example, the young 

 of our freshwater Unios (clams) soon after escaping from the mother 

 become attached to the gills of fishes for a period. In addition to being a 

 general method of migration, normal migration is with little doubt the 

 most effective means of dispersal, for it permits the species to become 

 acclimated to new conditions encountered. It is generally slow, but 

 when the age of the form is taken into consideration it is seen to be not 

 too slow to account for the present range of even the slow-moving species. 

 For example Gadow states that if a pair of earthworms are sufficient to 

 occupy one square yard of earth per year with their offspring, their de- 

 scendants long before 30,000 years (a not improbable estimate of the time 

 since the ice age) would have choked the whole earth if not repressed. 



