MIGRATIONS OF INSECTS. 265 



any of the surrounding fields. * An instance no less 

 marked occurs in the case of the marsh fritillary 

 (Melitcea artemis, Ochsenheimer), a butterfly so 

 very local, that, according to Harris, who observed 

 it at Wilsden, near Harrow-on-the-Hill, it seldom if 

 ever leaves the field in which it has been bred, 

 though hundreds of them may be seen there flying 

 low, and frequently settling. | It is probable the race 

 is now extinct at Wilsden, at least we have twice 

 failed in discovering them there at the season indi- 

 cated by Harris. 



These, however, are only exceptions to the general 

 rule of nature, which seems to be to diffuse a species 

 over as great a space as possible, and thus to stock 

 every corner of the earth with life and enjoyment. 

 Hence it is that while a very few species of moths 

 and butterflies are confined to certain fields, as a very 

 few species of fish are confined to certain lakes or 

 rivers, J by far the greater number may be seen wan- 

 dering from flower to flower, and from field to field, 

 with no other rule to direct their flight beyond the 

 most wayward caprice. All the movements of 

 insects, however, so far from being capricious, are 

 chiefly if not solely produced by the two greait prin- 

 ciples of self-preservation and reproduction, though 

 movements may sometimes be observed to which 

 neither of these principles very obviously apply — 

 anomalies which require other circumstances for 

 their explanation, as we shall now endeavour to ex- 

 emplify. 



Were it recorded that a numerous flight of spar- 

 rows directed their course in an undeviating straight 

 line towards the ocean, and not having sufficient 

 power of wing to cross it, were seen to drop into the 

 water and perish, it might well be doubted whether 



* J. R. t Harris, Aurelian, 28. 



t Mag. of Natural History, i, 487. 

 VOL. XII. 23 



