What I have just said, that the peculiar instinct and deceptive action we have been considering is made and kept bright by being bathed in blood, applies to all instinctive acts that tend to the preservation of life, both of the individual and species. Necessarily so, seeing that, for one thing, instincts can only arise and grow to perfection in order to meet cases which commonly occur in the life of a species. The instinct is not prophetic and does not meet rare or extraordinary situations. Unless intelligence or some higher faculty comes in to supplement or to take the place of instinctive action then the creature must perish on account of the limitation of instinct. Again, the higher and more complete the instinct the more perilous it is, seeing that its efficiency depends on the absolutely perfect health and balance of all the faculties and the entire organism. Thus, the higher instinctive faculty and action of birds for the preservation of the species, that of migration, is undoubtedly the most dangerous of all. It is so perfect that by means of this faculty millions and myriads of birds of an immense variety of species from cranes, swans, and geese down to minute goldcrests and firecrests and the smallest feeble-winged-leaf warblers, are able to inhabit and to distribute themselves evenly over all the temperate and cold regions of the earth, and even nearer the pole: and in all these regions they rear their young and spend several months each year, where they would inevitably perish from cold and lack of food if they stayed on to meet the winter. We can best realize the perfection of this instinct when we consider that all these migrants, including the young which have never hitherto strayed beyond the small area of their home where every tree and bush and spring and rock is familiar to them, rush suddenly away as if blown by a wind to unknown lands and continents beyond the seas to a distance of from a thousand to six or seven thousand miles; that after long months spent in those distant places, which in turn have grown familiar to them, they return again to their natal place, not in a direct but ofttimes by a devious route, now north, now north-east, now east or west, keeping to the least perilous lines and crossing the seas where they are narrowest. Thus, when the returning multitude recrosses the Channel into England, coming by way of France and Spain from north or south or mid-Africa and from Asia, they at once proceed to disperse over the entire country from Land's End to Thurso and the northernmost islands of Scotland, until every wood and hill and moor and thicket and stream and every village and field and hedgerow and farmhouse has its own feathered people back in their old places. But they do not return in their old force. They had increased to twice or three times their original numbers when they left us, and as a result of that great adventure a half or two-thirds of the vast army has perished.