RIGIDITY ACQUIRED WITH STRUCTURE. 51

and the consideration of these must now engage our attention.

It has already been shown that structure probably originates from the definite action of forces upon structureless living matter (Chapter IV.); and a simple illustration will show us how the acquirement of structure may influence the variability of the organism.

Let us suppose a body composed of numerous extremely mobile spherical particles; let us conceive that the definite action of energy sets all these particles vibrating in one direction, and that the spheres adhere to each other, and are ultimately converted into cylindrical rods, and these again, by mutual pressure, become hexagonal prisms. The extremely mobile body would be converted into one possessing increased mobility in the direction of the axes of the prisms; these would still be capable of free movement in the direction in which they were originally acted upon, but would not readily be altered by the impact of new forces acting in other planes. The prismatic structure of our hypothetical body might remain susceptible of great modification, but would surely exercise the strongest influence on all the subsequent changes which the body might undergo.

New structures might be added under the influence of new or modified forces; and these would

52 INFLUENCE OF LIGHT UPON THE RETINA.

act chiefly on particles the mobility of which remained free in the direction in which they acted. Such particles might arise from the accretion of new material; for new forces would act only slightly on the already acquired prismatic struc

ture.

We may thus see how the acquirement of structure may render organisms less susceptible to the action of new forces; and if this has been rendered more easy of conception by the above hypothetical illustration, it will have served its purpose; for the author disclaims all idea of pointing out the exact process by which rigidity has really been acquired; the principle is all that need be insisted on. It will probably be objected that the structure of organic beings is so complex that it is inconceivable that it can have originated and become fixed in this manner; but such an objection is not a weighty one, since it is impossible to conceive the extreme degree of susceptibility to the action of energy possessed by living matter. We know that light-rays of different colours affect the retina of the eye differently; that is, the susceptibility of its structure is such that it is affected by shocks of energy which are so minute and transient that six hundred and ninety-nine million millions occur in one second of time and the impression of violet is transmitted to the brain, whilst the impact of

EFFECT OF EXCESS OF FOOD.

53

about five hundred million million* vibrations gives the sensation of red; the other colours of the spectrum rise gradually in pitch from red to violet.

Every new structure acquired by an organism may be looked upon as a new means of narrowing the variability of the organism, and probably of increasing its susceptibility to the action of those forces by means of which such a structure is acquired, since so long as the increase of this susceptibility is useful to the organism, those forms which are most susceptible to such influences will survive.

If this view of the influence of structure on inheritance be correct, it is not difficult to see that variability may be connected with excess of food, as Andrew Knight supposes it to be: excess of food over that which is required to nourish already acquired structure would probably produce a fresh supply of undifferentiated structureless material; and this, on our hypothesis, would be susceptible to the action of new or modified forces.

Variation, however, is not only limited by structure, but also by the limitation of conditions. Thus the pebbles upon the beach vary considerably in size; but this variation is limited by the

* More exactly 474,439,680,000,000. Tyndall, ' Heat a Mode of Motion,' p. 243.

54 LIMITATION OF VARIABILITY BY CONDITIONS.

conditions under which they are formed: none are so large that they cannot be rolled by the action of the waves; and none are so small that they are lifted and dashed against each other by each successive breaker. In the first case the stones may be rounded and worn, but they do not assume the form of pebbles; and in the latter case the fragments are fractured into minute angular particles of sand.

We may fairly infer that the variability of structureless living protoplasm is limited by the conditions under which it is placed, and that it is further limited by the fact that protoplasm is extremely liable to undergo chemical change under altered conditions rendering it incapable of transforming energy; in other words, any considerable change produces death. It is probably only under gradually changing conditions that evolution progresses; and hence it is not difficult to understand why the lower forms of life remain constant, and transmit their characters truly through innumerable generations.

The accurate manner in which the form and markings of the beautiful geometrical flint shields of the Radiolaria and Diatomaceae (Plate III.) are inherited is not difficult to explain on this view, although at first sight it appears incredible that it should depend on so simple a law.

NODAL LINES OF VIBRATING SURFACES. 55

It is well known that if sand or fine dust be scattered upon a metal plate, and the plate be made to vibrate by means of a bow or a string drawn rapidly across one of its edges, the sand leaves certain portions of the plate and arranges itself in a beautiful geometrical figure. This is caused by the formation of centres of vibration in the plate, with lines, called nodal lines, between them, where the vibrations destroy each other and the particles of the plate are at rest. Nothing appears more probable than that similar points of vibration and rest exist upon the surface of these shield-forming organisms, and that the excreted silica which forms their shells comes to rest at the nodal points.

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Just as the particles of various gases are supposed to be continually vibrating, according to the kinetic theory, in waves of a definite length, which varies in different gases, so one can readily conceive the particles of extremely mobile protoplasm may vibrate regularly, and the nature of the vibrations may differ in the various species; this would determine the beautiful geometrical form of the flint shields diagnostic of each, by permitting the siliceous exudation to solidify only on the nodal lines.

It is a point worthy of careful consideration that the lowest forms of life only produce geo