most of their lives in places which seem to have been sought, at first, for shelter during the moulting period. A species of Porcellana clings to the lower surface of the broad shell of Limulus, and the Pinnixas live in the burrows which annelids make in the floor of the ocean. I have found a species of Pinnixa living on the shoals at Beaufort, N.C., in the parchment-like tubes with which the annelid Chatopterus lines its burrow, and as the opening of this tube is too small for a Pinnixa to pass, it must enter while small and pass the rest of its life there.

The period of moulting is dangerous, not only because of enemies, but also because of its critical nature, for many crabs die in the act, while others lose their limbs and their gills. The general constitutional disturbance is so great that it is difficult to carry a full-grown crab safely through it in an aquarium. The power to replace lost parts which is so well developed in crabs. is an adjustment to meet and compensate for this danger among others. Most of the direct danger comes from the stony hardness and inflexibility of the old shell, and the shells of crabs like the Pinnixa, and the female Pinnotheres which lives within the shell of the oyster, are softer than those of more exposed crabs.

The hermit-crabs and soldier-crabs live in the spiral shells of gasteropod mollusks, and, as these houses are strong enough to furnish ample protection, all the hinder part of the body of these crabs is covered by a thin flexible shell which may be stripped off without danger, although the claws and other exposed parts are covered by very hard strong shell. When born, the little hermit-crab is straight and its hind-body carries swimming feet, but when it is about as large as a mosquito, these become converted into knobs for clinging to the inside of the house, and the hindbody becomes twisted into a spiral to fit the inside of the spiral shell. Crabs outgrow the shells of mollusks just as children outgrow their clothes; and hermit-crabs are always on the watch for new shells, and exhibit what the human observer finds himself disposed to call a lively interest in shells. If half a dozen of them are placed in an aquarium, they soon begin to measure and compare shells, and even to make vacant one that seems eligible, by pulling out its occupant piece by piece and eating him. One

that has found or emptied a shell that seems to suit, measures it carefully inside and out, and then, bringing the openings close together, quickly pops out of the old into the new. Then the old shell is compared with the new, and often the body is slipped into each of them repeatedly, and each is allowed to slip nearly off and is then pulled on again, somewhat as a man settles himself into a new coat. Running is now tried in each shell, a claw keeping a tight clutch on the empty one and dragging it along; and the movement of drawing the body far into the shell, so that it drops on the sand as if it were empty, is tried in both. It is often many hours before a choice is made, and then the decision often is that the old one is best.

It is difficult to witness or to describe this performance without attributing to the crab feelings and motives like our own; yet, while no one can say whether the crab knows what it is about or not, nothing is more certain than that its actions are due to its nature, and not to knowledge of the value of a house, drawn from experience. When I was working as a student in the marine laboratory of Alexander Agassiz, he reared from eggs, in an aquarium, a brood of hermit-crabs which had never seen a shell. I had in my aquarium young gasteropods which I had reared from eggs. Some of them had died, and their empty shells were, at his suggestion, dropped into the water with the crabs, which seized them, almost as soon as they touched the water, and beginning to explore their interior as they were carried to the bottom by the weight of the shells, conducted themselves as if they had many years of experience in the use of molluscan shells as houses. I have seen very young hermit-crabs make houses for themselves out of the cast skins of others, although these afforded no protection; and I have found a full-grown one in the bowl of a clay pipe so badly broken that it exposed the soft abdomen and was useless; but the impulse to inhabit shells is almost universally protective and beneficial, although it is as strictly a part of the nature of hermit-crabs as is the twisted abdomen, or the legs and claws, or any other part of the crab's body.

The external world presents such variety that few natural adjustments are so exact and definite that they may not under some

circumstances prove disadvantageous or even destructive instead of beneficial, although the perfection of many of the adjustments of crustacea and insects is marvellous. Some hunting wasps store living spiders in the cells with their eggs to serve as food for their young, but each spider, while alive, is paralyzed and helpless, for when the wasp captures it she stings it through the nerve-centre which directs the movements of the limbs, severely enough to produce paralysis without destroying life; and Mivart says ("Lessons from Nature," p. 202) that the female wasp does this by nature or without experience.

It is often said that the natural activities of living things are innate; but, so far as this word implies that they take place without a stimulus, it is obviously erroneous. The hermit-crab is said to seek a house by nature, and the egg to grow into a specific organism in virtue of its inherent potency; but this is not strictly true, for while some vital changes may be spontaneous, in one of the many meanings of this word, there is no reason to believe that any change ever takes place, either in living things or any where else, without antecedents which stand in that peculiar relation which we call physical causation.

The new-born child is said to seek the breast instinctively, but every nurse knows that it does not seek the breast at all without experience, although it does suck by nature and without instruction the first time the nerves of its lips and tongue are stimulated by contact with the nipple. The instinct of the young hermit-crab cannot be called spontaneous, if, by this word, we mean arbitrary, although it is so promptly called forth by the first sight of a shell.

The bodily movements of which the purpose is most obvious are, as a rule, called out in response to changes in the external world, and they are excited by stimuli which come through the senses; although many responsive actions are called forth by stimuli which arise within the body and do not reach it through any of the organs of special sense, as the stretching of our limbs while awakening is excited by the vague discomfort of the body; and this is true not only of many bodily movements but of most physiological changes.

"To call mind a function of the brain," says Maudsley ("Responsibility in Mental Disease," p. 17), "may lead to much mis

apprehension if it be thereby supposed that the brain is the only organ which is concerned in the function of mind. There is not an organ in the body which is not in intimate relation with the brain by means of its paths of nervous communication, and which does not, therefore, affect more or less plainly and specifically its function as an organ of mind. It is not merely that a palpitating heart may cause anxiety and apprehension, or a disordered liver gloomy feelings, but there are good reasons for believing that each organ has a specific influence on the constitution and function of mind; an influence not yet set forth scientifically, because it is exerted on that unconscious mental life which is the basis of all that we consciously feel and think. Were the heart of one man," says Maudsley, "to be placed in the body of another, it would probably make no difference in the circulation of the blood, but it might make a real difference in the temper of his mind. So close is the physiological sympathy of parts in the commonwealth of the body, that it is necessary, in the physiological study of mind, to regard it as a function of the whole organism, as comprehending the whole bodily life.”

A most notable illustration of the way a complicated adaptive mechanism may be thrown into beneficial response by a physiological stimulus, is found in the shad, which, when its bodily structure is excited by the reaction of approaching sexual maturity, leaves its home in the ocean and enters upon a journey which, before its path was obstructed by dams, carried it across the broad states of Virginia, Maryland, and Pennsylvania, to its spawning ground in central New York.

The excitement of adaptive vital changes in one part of the body by changes in another part is not restricted to the channels afforded by the nervous system. Florists make their plants bloom before their time by confining their roots in small pots. The seeds of an apple are new beings, but the apple itself is part of the substance of the mother-tree, yet the blossoms will not set fruit unless they are fertilized.

When a duck's egg is put under a hen, it undergoes a long series of wonderful changes, which all prove, in the end, to be in responsive adjustment to the normal life of ducks; and as the production of a duck by the mere heat of a hen, or that of a lamp in an incu

bator, is incredible, we say the egg is developed by its inherent potency; but we must use these words with care, for the assertion that the changes which make up this long and marvellous series take place spontaneously is as incredible as the assertion that they are determined by the heat of the hen; and there is reason to believe that each change in the series transmits to the natural or inherent adaptive mechanism a stimulus which excites in it the performance of the responsive actions which bring about the next change in order.

Embryonic development is so delicate and so complicated that we cannot hope to trace, far less to imitate, the action of these stimuli in anything like their natural perfection; yet we can, now and then, rudely imitate some of them, while, in other cases, we can demonstrate their presence and influence indirectly by preventing them from acting. Some eggs which have begun their development by division into two, four, or eight cells, may be shaken apart without destroying their vitality, and when thus separated, a cell which would normally have given rise to half or quarter of an embryo, may give rise to a whole one of one half or one quarter the natural size. Embryologists are rapidly adding, by experimental methods, to our knowledge of the mechanics of development, and it has been known, since the day of Aristotle, that some of the latest stages in the development of the higher animals and of man do not take place in the absence of certain normal physiological stimuli.

Male mammals, for example, do not attain bodily perfection until the approach of sexual maturity. In man the beard begins to grow at what is accordingly called the age of pubescence; the larynx enlarges; the voice assumes a manly tone; the shoulders grow broad; the chest deepens; and the trunk and limbs begin to differ in relative length from those of women and children. At the same period in the life of a bull his neck and shoulders grow massive and sturdy; his forehead broadens and becomes cushioned with hair; and he becomes pugnacious and subject to fits of violent rage.

The cock acquires his spurs, his brilliant plumage and other ornaments, and begins to crow. Aristotle pointed out that when young male mammals or birds are prevented from becoming sexually mature, they fail to acquire the distinctive characteristics of their species, and this shows that the completion of this, the final stage