The defect of nest-building lies in the absence of protection for the young birds. When they grow large and feel strong they bubble, as it were, over the edge of the cup-shaped nest. Their wings, though not yet full-grown, save them from injury in descent by spreading out like a parachute, but are powerless to assist them after reaching the ground. In the grass they are the prey of rooks, crows, magpies, jackdaws, snakes, rats, and cats. They have no means of escape whatever: they cannot fly nor run--the tall grass stops running--and are frequently killed for amusement by their enemies, who do not care to eat them. Numbers die from exposure in the wet grass, or during rain, for they are not able to fly up and perch on a branch. The nest requires a structure round it like a cage, so that the fledglings might be prevented from leaving it till better able to save themselves. Those who go to South Kensington to look at the bird's-nest collection there should think of this if they hear any one discoursing on infallible instinct on the one hand, or evolution on the other. These two theories, the infallible instinct and that of evolution, practically represent the great opposing lines of thought--the traditional and the scientific. An examination of birds' nests, if conducted free of prejudice, will convince any independent person neither that the one nor the other explains these common hedge difficulties. Infallible instinct has not supplied protection for the young birds, nor has the experience of hundreds of years of nest-building taught the chaffinch or the missel-thrush to give its offspring a fair start in the famous 'struggle for existence.' Boys who want linnets or goldfinches watch till the young are almost ready to bubble over, and then place them in a cage where the old birds come and feed them. There is, then, no reason why the nest itself should not be designed for the safety of the fledgling as well as of the egg. Birds that nest in holes are frequently very prolific, notably the starling, which rears its brood by thousands in the hollow trees of forests. Though not altogether, in part their vast numbers appear due to the fact that their fledglings escape decimation.