Page:Popular Science Monthly Volume 71.djvu/76

70 impersonal and unconscious. It is not choice. Breeder's selection, on the other hand, is consciously directed towards a known and very definite end, the chosen "points." The action of natural selection is no more conscious than is the action of the current of water that separates pebbles from sand. This is the first great difference between natural selection and breeder's selection.

In another respect nature's agency in selection differs fundamentally from that of the breeder. The mode of operation of breeder's selection is positive; that of natural selection is negative. Natural selection eliminates by death the less well adapted members of a species. The better adapted survive and reproduce their kind. It does not matter in what respect they are better adapted. Protection from enemies is achieved in the case of the porcupine by his quills. The deer is saved by fleetness; wild cattle by the herding instinct, and by the effective use of horns and hoofs which that makes possible. No particular sort of quality is favored by natural selection, but those lacking in any respect are cut off. Nature has no plan. The line of evolution may take any direction; only, whatever the direction of improvement, woe to the hindmost. We have already seen that breeder's selection is conscious. That means its action is also positive. Attention is directed to reproducing and further evolving a favored type. The fan-tail pigeon exists because breeder's sought to develop a type with an unusually large number of tail feathers. The fleece of the better breeds of sheep has become fine and long because breeders sought this particular result. Breeder's selection positively favors certain individuals and types. Natural selection is primarily destructive of the inferior. It is negative. Incidentally it allows certain better adapted individuals to survive.

The third difference between natural selection and breeder's selection is that the latter operates directly on propagation, not necessarily by death. In "nature," this is among wild animals, the capacity to survive is the whole story. It may in general be assumed that a wild animal that survives to maturity, and lives through its prime, will reproduce its kind. Though it is the essential point always, propagation is not in general the crucial point with lower animals. Among