Some blind creatures live only in dark caves; the limpet only where it is alternately covered and uncovered by the tide; the red-snow alga rarely elsewhere than in the Arctic regions or among alpine peaks.

Among the forces external to the organism, and playing upon it, are heat and light, the elements contained in the water or air in which the organism lives, or the soil in which it may be rooted, the food taken into the stomach, the enemies with which it comes into collision, &c. Of course the external conditions could not do their work unless the organism were fitted to respondthere must be the malleable iron, as well as the hammer which strikes; and the iron re-acts on the hammer. As Mr G. H. Lewes says, the action of the medium on the organism is assuredly a potent factor, which biology cannot ignore; but the organism itself is a factor, and according to its nature the influence of the medium is defined. Moreover, if we fix attention on one part or organ of a living creature, all the other parts may be considered as external to it; every part influences every other part, as every particle of matter in the globe attracts every other particle, while still the globe as a whole has a gravitative relation to the sun and planets. Though every variation in an organ comes ultimately from the external conditions, the immediate cause is often to be sought in the other parts of the organism itself. "No one, I imagine," says Huxley, “would dream of seeking in the direct external conditions of his life for the cause of the development of the sixth finger and toe in the famous Maltese."

1 Fortnightly Review, Nov. 1868.

"2

2 Academy, Oct. 1869. Gratio Kelleia, the individual referred to, possessed six fingers on each hand and six toes on each foot; but there was nothing peculiar in his parents.

SUSCEPTIBILITY OF ORGANISMS.

95

All the forces without and within-all the forces of the universe-are indestructible forces; but they are correlated and interchangeable. Gravity or pressure can be changed into heat, heat into chemical affinity, chemical affinity into electricity. From electricity we can get magnetism, from magnetism mechanical motion, &c.; all are resolvable into motion, all are different modes of motion, the motion being indestructible, and its amount measurable.1 Therefore, force in any form acting on an organism may give rise to changes of relative position among its molecules, such as by accumulating will become palpable change of structure and function. Reasons are shown by Mr Spencer for believing that organic matter is built up of molecules, so very complex, and therefore so very unstable, that the slightest variation in their conditions destroys their equilibrium, and causes them either to assume altered structures or to decompose. But a substance which is, beyond all others, changeable by the actions and reactions of the forces liberated from instant to instant within its own mass, must be a substance that is, beyond all others, changeable by the forces acting on it from without. All the inward forces balance all the outward forces, not as opposing pressures produce fixity and stillness, but as the antagonist forces in the solar system preserve a moving equilibrium.

If, then, there exists this state of moving equilibrium among a definite set of internal actions, exposed to a definite set of external actions, what must result if any of the external actions are changed? Of course there is no longer an equilibrium. Some force which the organ

1 See Grove's Correlation of Forces, and Tyndall's Heat as a Mode of Motion.

ism habitually generates is too great or too small to balance some incident force, and there arises a residuary force exerted by the environment on the organism, or by the organism on the environment. This residuary force -this unbalanced force-of necessity expends itself in producing some change of state in the organism; and the change must be some alteration of functions, ending in the establishment of a new moving equilibrium, or else the organism will die.

The external forces, as already mentioned, are light, heat, food, &c. Living things have the power to imbibe water, and along with it the materials which work transformations; and through the agency of heat the water is evaporated, and makes room for a fresh supply. Light—which works those chemical changes utilized in photography, which causes the combinations of certain gases, and alters the molecular arrangement of many crystals-may be expected to produce marked effects on substances so complex and unstable as those which make up organic bodies. Experiments have shown that when the sun shines on living leaves they begin to exhale oxygen, and to accumulate carbon and hydrogen. -results which are traced to the decomposition by the solar rays of the carbonic acid and water absorbed. A permanent difference in the quantity of light or heat affects, day after day, the processes going on in the leaves. Habitual rain or drought alters all the assimilative actions, and appreciably influences the organs that carry them on. Some particular substance, by its presence in the soil, gives new qualities to some of the tissues, causing greater rigidity or flexibility, and so affecting the general aspect. Here, then, we have in plants changes tending to bring about modified arrange

CHANGES IN ORGANISMS.

97

ments of functions and structures, in equilibrium with modified sets of external forces. In terrestrial animals heat similarly aids the changes that are going on. The exhalations of vapour from the lungs and the surface of the skin, forming the chief escape of the water that is swallowed, conduces to the maintenance of those currents through the tissues, without which the functions would cease. For though the vascular system distributes nutritive fluids in ramified channels through the body, yet the absorption of these fluids into tissues partly depends on the escape of fluids which the tissues already contain. How readily vegetal and animal substances are modified by other substances put in contact with them we see illustrated every day. Besides the many compounds which cause the death of an organism into which they are put, we have the much greater number which work those milder effects termed medicinal; and most important of all, there are the substances which constitute food and the material for respiration. The substances of which the animal body is built up enter it in a but slightly oxidised and highly unstable state; while the great mass of them leave it in a fully oxidised and stable state. The union of oxygen with nitrogenous and other matters in the body produces animal heat; and just as in a burning piece of wood the heat given out by the portion actually combining with oxygen raises the adjacent portion to a temperature at which it also can combine with oxygen; so in a living animal the heat produced by oxidation of each portion of tissue maintains the temperature at which the unoxidised portions can be readily oxidised. In addition to heat, light, humidity, food taken into the stomach, and air breathed by the lungs or gills, animals are affected

G

by the exercise they give their bodies, the strain when they struggle with foes, the mechanical pressure of external substances, &c., &c.

To illustrate the way in which Natural Selection may be supposed to work in connexion with the external conditions which first of all give rise to the variations, let us take a simple case. The callosities on the knuckles of the gorilla are adapted to that creature's habit of partially supporting itself on its closed hands when moving along the ground, as the horny hand of a workman is adapted to his continual use of a hammer. The gorilla will transmit the callosities to its offspring, and the individuals in which they are the most developed will have the better chance so far in the struggle for life. But the contact of the knuckles with the ground is fitted to cause hardness of skin, and is probably the external condition which, in the course of many generations, has actually produced the callosities. With the first gorillas who resorted to that mode of progression the result might be trifling; but if it were in any degree inherited, together with the habit of walking in that way, the second generation would add to it, and so on till it became what it is.

(2.) Outward forces vary.-But all these conditions of a creature's environment are continually altering, or the organism is thrust by pressure of population into a new habitat. Besides the variations in the daily supply of light and heat, and further variations in the annual supply-all responded to by variations in the functions of living things-there are variations which complete themselves in 21,000 years, through the altered position of the earth's poles with respect to the sun; and variations, whose cycle is millions of years, through the changing