In other words, there are hereditary pre-arrangements of nerve-cells and muscle-cells which come into activity almost as easily as the beating of the heart. In a minute or two the newborn pigling creeps close to its mother and sucks milk. It has not to learn how to do this any more than we have to learn to cough or sneeze. Thus animals have many useful ready-made, or almost ready-made, capacities of doing apparently clever things. In simple cases of these inborn pre-arrangements we speak of reflex actions; in more complicated cases, of instinctive behaviour. Now the caution is this, that while these inborn capacities usually work well in natural conditions, they sometimes work badly when the ordinary routine is disturbed. We see this when a pigeon continues sitting for many days on an empty nest, or when it fails to retrieve its eggs only two inches away. But it would be a mistake to call the pigeon, because of this, an unutterably stupid bird. We have only to think of the achievements of homing pigeons to know that this cannot be true. We must not judge animals in regard to those kinds of behaviour which have been handed over to instinct, and go badly agee when the normal routine is disturbed. In ninety-nine cases out of a hundred the enregistered instinctive capacities work well, and the advantage of their becoming stereotyped was to leave the animal more free for adventures at a higher level. Being "a slave of instinct" may give the animal a security that enables it to discover some new home or new food or new joy. Somewhat in the same way, a man of methodical habits, which he has himself established, may gain leisure to make some new departure of racial profit. [Illustration: _Photo: O. J. Wilkinson._ JACKDAW BALANCING ON A GATEPOST The jackdaw is a big-brained, extremely alert, very educable, loquacious bird.] [Illustration: _From Ingersoll's "The Wit of the Wild."_ TWO OPOSSUMS FEIGNING DEATH The Opossums are mainly arboreal marsupials, insectivorous and carnivorous, confined to the American Continent from the United States to Patagonia. Many have no pouch and carry their numerous young ones on their back, the tail of the young twined round that of the mother. The opossums are agile, clever creatures, and famous for "playing 'possum," lying inert just as if they were dead.] [Illustration: MALE OF THREE-SPINED STICKLEBACK, MAKING A NEST OF WATER-WEED, GLUED TOGETHER BY VISCID THREADS SECRETED FROM THE KIDNEYS AT THE BREEDING SEASON] [Illustration: A FEMALE STICKLEBACK ENTERS THE NEST WHICH THE MALE HAS MADE, LAYS THE EGGS INSIDE, AND THEN DEPARTS In many cases two or three females use the same nest, the stickleback being polygamous. Above the nest the male, who mounts guard, is seen driving away an intruder.]