IN VERTEBRATES 61 



This change of habit, of course, took place in the remote 

 past, but the following very interesting modern example of pre- 

 cisely similar kind is given by Headley (in The Structure and 

 Life of Birds)'. " The Palm Swift in Jamaica till 1854 always 

 built in palms. But in Spanish Town, when two cocoa-nut palms 

 were blown down, they drove out the Swallows from the Piazza 

 of the House of Assembly and built between the angles formed 

 by the beams and joists." Of other such cases Newton thus 

 writes (in A Dictionary of Birds)'. "But though in a general 

 way the dictates of hereditary instinct are rigidly observed by 

 Birds, in many species a remarkable degree of elasticity is 

 exhibited or the rule of habit is rudely broken. Thus, the 

 noble Falcon, whose ordinary eyry is on the beetling cliff, 

 will for the convenience of procuring prey condescend to lay 

 its eggs on the ground in a marsh, or appropriate the nest of 

 some other bird in a tree. The Golden Eagle, too, remark- 

 ably adapts itself to circumstances, now rearing its young on a 

 precipitous ledge, now on the arm of an ancient monarch of the 

 forest, and again on a treeless plain, making a humble home 

 amid grass and herbage. Herons also show the same versa- 

 tility, and will breed according to circumstances in an open fen, 

 on sea-banks, or (as is most usual) on lofty trees. Such changes 

 are easy to understand. The instinct of finding food for the 

 family is predominant, and where most food is, there will the 

 feeders be gathered together. This explains, in all likelihood, 

 the associated bands of Ospreys or Fish- Hawks, which in North 

 America breed (or used to breed) in large companies where 

 sustenance is plentiful, though in the Old World the same species 

 brooks not the society of aught but its mate." 



MIGRATION OF BIRDS. Nothing can be more familiar than 

 the fact that innumerable species of birds undertake periodic 

 journeys, often of extreme length, from one region to another, 

 and at the same time nothing in the entire realm of natural 

 history is more mysterious. Broadly speaking, the same migrant 

 species has its own line of travel between its two places of 

 residence. The Golden Plovers, for example, of the northern 

 part of North America, fly south to the north of South America 

 via the Bermudas and Antilles. The paths of a number of 

 species are more or less coincident, in many cases, to form 

 what is known as a " migration route", and some of these routes 



