116 STUDIES IN LIFE AND SENSE. 



the migrations of higher forms of life. The swarms of locusts which 

 from time to time visit regions in which they are total or comparative 

 strangers, probably follow some law or habit of the kind under con- 

 sideration. Travellers have placed on record the interesting fact 

 that hordes of butterflies occasionally pass from one district or region 

 to another; this insect stream, numbering its gaudy members by 

 thousands, pursuing its course for days without cessation. In such 

 a case, the cause of migration is utterly unknown ; and, as in other 

 cases of animal journeyings, the somewhat irregular and erratic 

 nature of the habit only tends to render its elucidation the more 

 difficult The little beetles well known under the name of ladybirds, 

 .valued by the gardener as the enemies of the plant-lice, are known 

 occasionally to appear in some districts in immense swarms, and to 

 disappear as suddenly and as mysteriously as they appear. Regard- 

 ing the journeyings of insects from one country or region to another 

 there is an obvious difficulty of accounting for their movements 

 when we consider that neither considerations connected with food 

 and its scarcity, the breeding habits of the animals, or other condi- 

 tions of life, will account for their migration. Probably it might be 

 the more warrantable course to regard such journeyings as accidental 

 to a large extent, and as therefore partaking less of the true nature 

 of migration than instances where a regular and periodically recurring 

 journey is made from one country to another. 



Cases of migration which have been, and still are, determined by 

 the recurrence of the breeding season and reproductive habits of the 

 animals concerned, are exemplified by certain fishes. Of these the 

 salmon, and herring are the best known, but the list might be aug- 

 mented by the addition of the mackerel, pilchards, cod, and other of 

 our food-fishes. The migrations of the salmon are thus perfectly 

 ascertained to correspond not merely with its gradual growth towards 

 maturity, but with the deposition of its ova or eggs. These fishes 

 ascend rivers to spawn, and then migrate once more to the sea. 

 They alternate in this way between fresh and salt water, not merely 

 during adult life, but also during their earlier stages of existence. As 

 the " parr," or form in which it leaves the egg, the young salmon in- 

 habits fresh water ; whilst as the silvery " smolt," which succeeds 

 the parr-epoch, it seeks the sea. So thoroughly necessary for the 

 growth and life of the smolt is the seaward migration, that the 

 " grilse " stage, or subsequent epoch of salmon-growth, is completely 

 delayed unless the smolt is allowed to pass to the briny deep ; whilst 

 the " parr," on the other hand, languishes and dies if placed in sea 

 water. Rapid growth succeeds the seaward journey ; the fish migrat- 

 ing, as the " grilse," to fresh water to spawn, and year by year 

 repeating this latter journey as the salmon. That such migration is 

 something more than a casual habit appears to be proved by the fact 



