380 MENTAL EVOLUTION IN ANIMALS. 



stant struggle going on throughout nature between the 

 instinct of the one to escape its enemy and of the other to 

 secure its prey. If the instinct of the Spider be admirable, 

 that of the Fly which rushes into its toils is so far inferior. 

 Bare and occasional sources of danger are not avoided : if 

 death inevitably ensues, and creatures cannot have learnt by 

 seeing others suffer, it seems that no guardian instinct is 

 acquired : thus the ground within a solfortara in Java is 

 strewed with the carcases of tigers, birds, and masses of 

 insects killed by the noxious exhalations, with their flesh, 

 hairs, and feathers preserved, but their bones entirely con- 

 sumed.* Migratory instinct not rarely fails, and the animals, 

 as we have seen, are lost. What ought we to think of the 

 strong impulse which leads Lemmings, Squirrels, Ermines,t 

 and many other animals which are not regularly migratory, 

 occasionally to congregate and pursue a headlong course, 

 across great rivers, lakes, and even into the sea, where vast 

 numbers perish ; and ultimately it would appear that all 

 perish ? The country being overstocked seems to cause the 

 original impulse ; but it is doubtful whether in all cases 

 scarcity actually prevails. The whole case is quite inex- 

 plicable. Does the same feeling act on these animals which 

 causes men to congregate under distress and fear; and are 

 these occasional migrations, or rather emigrations, a forlorn 

 hope to find a new and better land ? The occasional emigra- 

 tions of insects of many kinds associated together, which, as 

 I have witnessed, must perish by countless myriads in the 

 sea, are still more remarkable, as they belong to families none 

 of which are naturally social or even migratory.^ 



* Von Buch, Descript. Phys. des lies Canaries, 1S36, p. 423, on the 

 excellent authority of M. fieinwardts. 



+ L. Lloyd, Scandinavian Adventure, 1854, vol. ii, p. 77, gives an excellent 

 account of the migration of Lemmings : when swimming across a lake, if 

 they meet a boat, they crawl up one side and down the opposite side. Great 

 migrations took place in 1789, 1807, 1808, 1813, 1823. Ultimately all seem 

 to perish. See Hogstrom's account in Stcedish Acts, vol. iv, 1763, of ermines 

 migrating and entering the sea. See Bachman's account in Mag. of Nat. Hist., 

 N.S., vol. iii, 1839, p. 229, of the migration of squirrels; they are bad 

 swimmers and get across great rivers. 



X Mr. Spence in his Anniversary address to the Entomological Society, 

 1848, has some excellent remarks on the occasional migration of insects, and 

 shows how inexplicable the case is. See also Kirby and Spence, Entomology, 

 vol. ii, p. 12; and Weisscnborn in Mag. of Nat. Hist., N.S., 1834, vol. iii, 

 p. 51G, for interesting details on a great migration of Libclluloe, generally 

 along the course of rivers. 



