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 {114} 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 {115} 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.