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 Zhe 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.” 
Micration or Brrps.—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 
