GEOGRAPHICAL DISTRIBUTION 255 



the wetness will likewise restrict population increase. By 

 combining data respecting temperature and moisture in con- 

 junction with the prevalence of soils of the right type, regions 

 favourable to the incidence of the insect appear to be tolerably 

 clearly explained. 



The life-history of the Colorado beetle {Leptinotarsa decemlineata) 

 affords an interesting ecological study. Its original home was 

 in the Rocky Mountains region, where its native food-plant is 

 the buffalo bur (Solanum rostratum). Following the westward 

 march of civilisation in North America came the potato, and the 

 insect found in this new food-plant one admirably suited to its 

 economy. The subsequent history of the species shows that it 

 has extended its range over a vast territory, which it did not 

 previously inhabit, owing to the presence of the new plant-host. 

 The insect was detected, for example, in Nebraska in 1859, and by 

 1874 it had reached the Atlantic coast. On the other hand, its 

 spread towards the Pacific slope has been very slow, owing to the 

 barrier of the Rocky Mountains : it did not occur in British 

 Columbia until 1919, and its presence in the states of Washington 

 and Oregon is likewise comparatively recent, and there does not 

 appear to be any evidence that it crossed the mountain range 

 by its own unaided activities. It would seem that the original 

 restricted distribution of this insect has to be ascribed largely to 

 biological causes and not to climatic influences. Once suitable 

 food-plants, such as the potato, tobacco and others, became 

 available in new areas, the impetus to spread was imparted. The 

 history of its diffusion shows that climatic factors have played a 

 relatively minor part in the process, since its range to-day is from 

 Mexico in the south to Quebec in the north. This territory lies 

 between the North American January isotherms of 10° and — 12° 

 and between the July isotherms of 30° and 20°. The annual 

 rainfall over the whole infested area varies from about 15 inches 

 in the north-west to some 60 inches in the south. It would appear 

 obvious, therefore, that the Colorado beetle can withstand a wide 

 range of climates before meeting such extremes as are likely to 

 become effective as limiting factors. In this connection its 

 accidental entry into France in 1922 is of particular importance. 



