MIGRATION IN ITS BEARING ON LAMARCKISM 109 



potamia and Asia Minor and the Roman Empire attracted the 

 Tartars and Scythians and Goths from their sterile and desolate 

 northern lands into the fertile homes of southern civilization. 

 Their journeys are no doubt initiated by an unconscious impulse, 

 which, before it brought them into contact with man, was useful 

 in some way to the species ; and this seems to be true also of the 

 migrations of certain prolific species of grasshoppers and locusts 

 which, inhabiting sandy and sterile regions, often overflow the 

 limits of their natural home, and invade more fertile regions 

 where they are not usually found. While there is no reason to 

 suppose that these movements are undertaken through a deliber- 

 ate intention to find new feeding grounds, lack of food is no 

 doubt the chief factor in the development of the migratory 

 instinct of rodents, as well as that of grasshoppers and locusts, 

 which resemble birds in their ability to make long journeys on 

 the wing without rest. The African locust has been met with at 

 sea, in great clouds, more than twelve hundred miles from land, and 

 the species sometimes wanders from its home in Africa to England. 

 While the movements of rodents and insects show that the 

 search for food has much to do with migration, they lack most 

 of the features which make the migration of birds so remarkable. 

 They occur at irregular intervals, while the movements of birds 

 are almost as regular as the almanac ; for while sea-birds seem 

 much exposed to storms, the days of their arrival and departure 

 may be predicted with as much certainty as if they were satellites 

 revolving around the earth. " Foul weather and fair, hot or cold ; 

 the pufifins make their appearance at the proper day as promptly 

 as if they were moved by clock work." While the course of the 

 migration of rodents and locusts is determined by conditions so 

 complicated and irregular that they may be called accidental, the 

 northward journey of birds is often directed to a definite spot 

 thousands of miles from the starting-point, and the resemblance 

 between irregular migrations in search of food and the migrations 

 of birds is too imperfect to tell us much about the latter, which 

 is much more like the movements of certain fishes like the shad, 

 which at a definite season enters upon a journey along a definite 

 path to a spot hundreds of miles away, to return again after the 

 purpose of the journey is accomplished. 



