[7] See J. Arthur Thomson's Progress of Science in the Nineteenth Century, 1903, p. 317, and E. B. Wilson's The Cell in Development and Inheritance, 1900.
Waste and Repair.—Organisms are systems for transforming matter and energy and the law of conservation holds good. "Each portion of mechanical or other energy which an organism exerts implies the transformation of as much organic matter as contained this energy in a latent state," and the waste must be made good by repair. We thus see why plants with an enormous income of energy and little expenditure of energy have no difficulty in sustaining the balance between waste and repair; we understand the relation between small waste, small activity, and low temperature in many of the lower animals; we understand conversely the rapid waste of energetic, hot-blooded animals. The deductive interpretation of waste is easy, but it is different with repair, for here the analogy between the organism and an inanimate engine breaks down. The living creature is a self-stoking, self-repairing, and also—it may be noted in passing—a self-reproducing engine. Spencer did not do more than restate the difficulty when he said that the component units of organisms have the power of moulding fit materials into other units of the same order.
In passing to consider the ability which an organism often has of recompleting itself when one of its parts has been cut off, just as an injured crystal recompletes itself, Spencer was led to the hypothesis that "the form of each species of organism is determined by a peculiarity in the constitution of its units—that these have a special structure in which they tend to arrange themselves; just as have the simpler units of inorganic matter." "This organic polarity (as we might figuratively call this proclivity towards a specific structural arrangement) can be possessed neither by the chemical units nor the morphological units, we must conceive it as possessed by certain intermediate units, which we may term physiological." But if in each organism the physiological units which result from the compounding of highly compound molecules have a more or less distinctive character, the germ-cell is not so very indefinite after all.
Many of the facts of regeneration are very striking. A crab may regrow its complex claw, a starfish arm may regrow an entire body. A snail has been known to regenerate an amputated eye-bearing horn twenty times in succession, a newt can replace a lost lens, a lizard can regrow its tail and part of its leg, a stork can regrow the greater part of its bill. In many cases, the surrender of parts which are afterwards regrown is exceedingly common, as in some worms and Echinoderms, and is a life-saving adaptation. Organically, though not consciously, the brainless starfish has learned that it is better that one member should perish than that the whole life should be lost. This regenerative capacity no doubt implies certain properties in the living matter and in the organism, but we are far from being able to picture how it comes about. What does seem clear is that the distribution and mode of occurrence of the regenerative capacity—in external organs often, but in internal organs very rarely; in most lizard's tails, but not in the chamæleon's; in the stork's bill but not in its toes—are adaptive, being related to the normal risks of life, as Réaumur, Lessona, Darwin, and Weismann have pointed out. According to Lessona's Law, which Weismann has elaborated, regeneration tends to occur in those organisms and in those parts of organisms which are in the ordinary course of nature most liable to injury. To which we must add two saving-clauses—(a) provided that the lost part is of some vital importance, and (b) provided that the wound or breakage is not in itself very likely to be fatal. In Weismann's words, the theory is, that "the power of regeneration possessed by an animal or by a part of an animal is regulated by adaptation to the frequency of loss and to the extent of the damage done by the loss."
Adaptation.—Wherever we look in the world of organisms we find examples of adaptation; we see form suited for different kinds of motion, organs suited for their uses, constitution suited to circumstances in such external features as colouring and in such internal adjustments as the regulation of temperature; we find effective weapons and effective armour, flowers adapted to insect visitors and insect visitors adapted to flowers, one sex adapted in relation to the other, the mother adapted to bearing and rearing offspring, the embryo adapted to its pre-natal life; everywhere there is adaptation in varying degrees of perfection. The adaptation is a fact, in regard to which all naturalists are agreed; difference of opinion arises when we ask how these adaptations have come to be.
In the chapter "Adaptation" Spencer practically restricted his attention to a certain kind of adaptation, namely the direct modifications which result from use or disuse, or from environmental influence. The blacksmith's arm, the dancer's legs, the jockey's crural adductors, illustrate direct results of practice; "à force de forger on devient forgeron." The skin forms protective callosities where it is much pressed or rubbed, as on the schoolboy's hands or the old man's toothless gums. The blood-vessels may respond by enlargement to increased demands made on them; the fingers of the blind become extraordinarily sensitive.
Spencer points to the general truth that extra function is followed by extra growth, but that a limit is soon reached beyond which very little, if any, further modification can be produced. Moreover, the limited increase of size produced in any organ by a limited increase of its function, is not maintained unless the increase of function is permanent. When the modifying influence is removed, the organism rebounds or tends to rebound. A lasting change of importance involves a re-organisation, a new state of equilibrium.
On inductive and deductive grounds, Spencer summed up in four conclusions:—
(1) An adaptive change of structure will soon reach a point beyond which further adaptation will be slow.
(2) When the modifying cause has been but for a short time in action, the modification generated will be evanescent.
(3) A modifying cause acting even for many generations will do little towards permanently altering the organic equilibrium of a race.
(4) On the cessation of such cause, its effects will become unapparent in the course of a few generations.