In the spiritual world, also, the subtle influences which form and transform the soul are Heredity and Environment. And here especially where all is invisible, where much that we feel to be real is yet so ill-defined, it becomes of vital practical moment to clarify the atmosphere as far as possible with conceptions borrowed from the natural life. Few things are less understood than the conditions of the spiritual life. The distressing incompetence of which most of us are conscious in trying to work out our spiritual experience is due perhaps less to the diseased will which we commonly blame for it than to imperfect knowledge of the right conditions. It does not occur to us how natural the spiritual is. We still strive for some strange transcendent thing; we seek to promote life by methods as unnatural as they prove unsuccessful; and only the utter incomprehensibility of the whole region prevents us seeing fully--what we already half-suspect--how completely we are missing the road. Living in the spiritual world, nevertheless, is just as simple as living in the natural world; and it is the same kind of simplicity. It is the same kind of simplicity for it is the same kind of world--there are not two kinds of worlds. The conditions of life in the one are the conditions of life in the other. And till these conditions are sensibly grasped, as the conditions of all life, it is impossible that the personal effort after the highest life should be other than a blind struggle carried on in fruitless sorrow and humiliation.
Of these two universal factors, Heredity and Environment, it is unnecessary to balance the relative importance here. The main influence, unquestionably, must be assigned to the former. In practice, however, and for an obvious reason, we are chiefly concerned with the latter. What Heredity has to do for us is determined outside ourselves. No man can select his own parents. But every man to some extent can choose his own Environment. His relation to it, however largely determined by Heredity in the first instance, is always open to alteration. And so great is his control over Environment and so radical its influence over him, that he can so direct it as either to undo, modify, perpetuate or intensify the earlier hereditary influence within certain limits. But the aspects of Environment which we have now to consider do not involve us in questions of such complexity. In what high and mystical sense, also, Heredity applies to the spiritual organism we need not just now inquire. In the simpler relations of the more external factor we shall find a large and fruitful field for study.
The influence of Environment may be investigated in two main aspects. First, one might discuss the modern and very interesting question as to the power of Environment to induce what is known to recent science as Variation. A change in the surroundings of any animal, it is now well-known, can so react upon it as to cause it to change. By the attempt, conscious or unconscious, to adjust itself to the new conditions, a true physiological change is gradually wrought within the organism. Hunter, for example, in a classical experiment, so changed the Environment of a sea-gull by keeping it in captivity that it could only secure a grain diet. The effect was to modify the stomach of the bird, normally adapted to a fish diet, until in time it came to resemble in structure the gizzard of an ordinary grain-feeder such as the pigeon. Holmgren again reversed this experiment by feeding pigeons for a lengthened period on a meat-diet, with the result that the gizzard became transformed into the carnivorous stomach. Mr. Alfred Russel Wallace mentions the case of a Brazilian parrot which changes its color from green to red or yellow when fed on the fat of certain fishes. Not only changes of food, however, but changes of climate and of temperature, changes in surrounding organisms, in the case of marine animals even changes of pressure, of ocean currents, of light, and of many other circumstances, are known to exert a powerful modifying influence upon living organisms. These relations are still being worked out in many directions, but the influence of Environment as a prime factor in Variation is now a recognized doctrine of science.[81]
Even the popular mind has been struck with the curious adaptation of nearly all animals to their _habitat_, for example in the matter of color. The sandy hue of the sole and flounder, the white of the polar bear with its suggestion of Arctic snows, the stripes of the Bengal tiger--as if the actual reeds of its native jungle had nature-printed themselves on its hide;--these, and a hundred others which will occur to every one, are marked instances of adaptation to Environment, induced by Natural Selection or otherwise, for the purpose, obviously in these cases at least, of protection.