living bone transplanted to fleshy parts of animals where there was no bone, continued to grow for a time by a life of their own, and increased by formation of additional bone, like a crystal grows in its medium; but after a time they diminished and disappeared. In a similar manner we are all of us aware of the persistency of ideas, even in opposition to the will, after the cause of them has been removed. Sometimes we cannot retain an idea because of the persistence of others; and at other times we cannot get rid of one, for a similar reason. Our mental habits also have often very great persistence.
The principle of heredity may be viewed as a result of the First law of Motion, and appears as Persistency of state, either of structure, form, or mode of action. It appears both in inanimate bodies, living structures, and in mental phenomena; in the latter, as hereditary mental peculiarities. The principle of Persistency of structure and Heredity of form and property, during repeated or even continual dissolution and aggregation of a material substance, is more or less manifest nearly throughout the whole of nature. In the formation of crystals it is clearly seen; each crystallizable substance will only grow into its own shape or shapes; each particle of common salt, during an endless series of successive solutions and aggregations into the solid state, always forms a more or less perfect cube; that of silica a hexagon; and so on throughout the entire series of thousands of different crystalline bodies. As each
form of crystal only produces crystals of like form and property (or at most in certain cases a limited number of modified forms, as in the instance of calcic carbonate, &c.) so also each seed, both of animals and vegetables, only produces its own particular essential shape and collection of functions. The same principle shews itself in the transmission of particular types of disease, and of eccentricities of organization, from one generation to another of animals. Peculiar malformations of body and characteristics of mind often persist in families from generation to generation. This persistency or heredity of structure and of property is not limited to solid bodies, but exists also in liquids: "The effect of vaccine virus upon the liquid blood, in producing a permanent and organic change in its constitution and character, which continues to exercise a protective influence against small-pox, in the great mass of cases, through a long life, during which time the blood must have undergone, many thousands, if not millions of changes and modifications." (F. Winslow. "Obscure diseases of Brain and Mind," page 432). The same persistency of structure and property of structure, has even been detected in vapours; the vapour of red iodide of mercury for example, deposits only crystals of red iodide, whilst that of the yellow deposits only yellow (see Gmelin's Handbook of Chemistry, vol. 1, p. 100.)
We often appear to mentally select when we only yield to causes acting upon us, i.e., to the strongest influence or motive. That "self-preservation is the
first law of nature," is not only true of living creatures, but largely also of dead substances. Inanimate as well as animate matter, appears to usually select what is good for itself. Apparent selection, which is manifested in the phenomena of instinct, is exhibited not only by brain, but by all material substances. Acids appear to select bases, North magnetism rejects North and prefers South magnetism. Also if a piece of zinc is put into a mixed solution of the nitrates of silver, magnesium, calcium, strontium, barium, lithium, sodium, potassium, and rubidium, it will select the silver only with which to form a "metallic tree," and reject all the other metals. Everything which aggregates or grows to a definite shape, appears to select its material; if a crystal of a particular salt is placed in a mixture of saturated solutions of different salts, it will only select and assimilate to itself suitable material, either particles of the same composition as itself, or those which are isomorphous with it, i.e. belonging to the same crystalline system. In living bodies also, the same principle operates; Living tissues, whether of animals or vegetables, usually select from their nutrient fluids, and assimilate, particles only of those kinds of matter which are suitable for their structure; in this way, a bone assimilates lime and phosphoric acid from the multitude of different substances conveyed to it by the blood. And in all these cases, the selecting material appears to act as if it possessed the powers of instinct, perception, comparison, judgment, and
volition. The act of self-repair is clearly connected with this, and is not limited to living structures; Sir David Brewster observed that if a portion of the surface of a perfect crystal of alum is very slightly abraded by dissolving a film from it, and the crystal be then immersed during a very brief period in a saturated solution of alum, the abraded portion repairs itself. The subjects of "malformation of crystals," and "diseases of crystals" have been scientifically investigated. The power of selection (or rather of apparent selection) is no doubt a result of the combined action of causation and of the inherent properties of bodies, and depends, like consciousness, upon difference of impression, the strongest suitable influence determining. If apparent selection can thus be performed by inanimate matter, we should not, except for a very sufficient reason, assume the existence in living creatures, of a special occult power to perform the same function. In the selection of ideas also the intellect acts according to the purely scientific method.
We frequently appear to mentally adapt ourselves to particular circumstances when we are really determined by causes; and this apparent adaptation is also seen in ordinary physical and chemical phenomena. The course of a river for instance, adapts itself to the configuration of the country through which it flows, and if it cannot pass wholly by one channel, as in seasons of flood, or on occasions of accidental obstruction, it travels through several; and a similar result
occurs with the flow of the blood when an artery is tied or becomes obstructed. A plant when growing in a dark recess, bends itself towards the light as if it preferred light; and its roots adapt themselves to the forms of existing obstacles. A decapitated frog jumps away from a source of irritation, as if he still possessed sensation, volition, and choice. A man seeking his way through a crowd avoids the course in which the throng is densest. The human mind also, chooses as it were, the easiest way of solving a problem, and usually adapts itself to altered circumstances.
The principle of evolution also operates both in physical and mental actions, and is a result of that of causation. Complexity of structure and function is evolved out of simplicity of composition and property by plurality of causes and conditions. For instance, many complex forms of crystals of ice are produced from water. Calcspar crystallizes in more than one hundred varieties of form, (all derived from an obtuse rhombohedron) under the influence of a number of slightly different conditions of temperature, impurities in the solution, &c. The most complex bodies are evolved out of the simplest, the bodily frame of man himself (and that of other animals) is constructed of less than twenty of the elementary substances. The same simple substances are capable of yielding very different and more complex bodies under different conditions; thousands of different chemical compounds are composed of hydrogen, oxygen, and carbon only. In the development of
living forms from ova, the ultimate form produced does not exist in the germ, any more than a crystal exists in its solution, but is a natural consequence of the forces acting in and upon the germ, like the cubical form of a crystal of common salt is a result of the forces acting in its constituents under the conditions of its environment, especially those of pressure and temperature. The extent to, and manner in which, the force and principle operate, depend upon the material substance, and its conditions internal and external.