CHAPTER XIX THE INFLUENCE OF HABITS AND SURROUNDINGS 1. The Influence of Function—2. The Influence of Surroundings— 3. Our own Environment Disuse 1. The Influence of Function. - -A skilled observer can often discern a man's occupation. from his physiognomy, his shoulders, or his hands. In some unhealthy occupations the death-rate is three times that in others. of such organs as muscles tends to their degeneration, for the nerves which control them lose their tone and the circulation of blood is affected; while on the other hand increased exercise is within certain limits associated with increased development. A force de forger on devient forgeron. If we knew more about animals we might be able to cite many cases in which change of function produced change of structure, but there are few careful observations bearing on this question. Even if we could gather many illustrations of the influence of use and disuse on individual animals, we should still have to find out whether the precise characters thus acquired by individuals were transmissible to the offspring, or whether any secondary effects of the acquired characters were transmissible, or whether these changes had no effect upon succeeding generations. As there are few facts to argue from, the answers given to these questions are not reliable. It is easy to find hundreds of cases in which the constant characters of animals may be hypothetically interpreted as the result of use or disuse. Is the torpedo-like shape of swift swimmers due to their rapid motion through the water, do burrowing animals necessarily become worm-like, has the giraffe lengthened its neck by stretching it, have hoofs been developed by running on hard ground, are horns responses to butting, are diverse shapes of teeth the results of chewing diverse kinds of food, are cave-animals blind because they have ceased to use their eyes, are snails lopsided because the shell has fallen to one side, is the asymmetry in the head of flat fishes due to the efforts made by the ancestral fish to use its lower eye after it had formed the habit of lying flat on the bottom, is the woodpecker's long tongue the result of continuous probing into holes, are webbed feet due to swimming efforts, has the food-canal in vegetarian animals been mechanically lengthened, do the wing bones and muscles of the domesticated duck compare unfavourably with those of the wild duck because the habit of sustained flight has been lost by the former ? But these interpretations have not been verified; they are only probable. "It is infinitely easy," Semper says, "to form a fanciful idea as to how this or that fact may be hypothetically explained, and very little trouble is needed to imagine some process by which hypothetical fundamental causes-equally fanciful-may have led to the result which has been actually observed. But when we try to prove by experiment that this imaginary process of development is indeed the true and inevitable one, much time and laborious research are indispensable, or we find ourselves wrecked on insurmountable difficulties.” Not a few naturalists believe in the inherited effects of functional change mainly because the theory is simple and logically sufficient. If use and disuse alter the structure of individuals, if the results are transmitted and accumulate in similar conditions for generations, we require no other explanation of many structures. The reasons why not a few naturalists disbelieve in the inherited effects of functional change are (1) that definite proof is wanting, (2) that it is difficult to understand how changes produced in the body by use or disuse can be transmitted to the offspring, (3) that the theory of the accumulation of (unexplained) favourable variations in the course of natural selection seems logically sufficient. I should suspend judgment, because it is unprofitable to argue when ascertained facts are few. But if you like to argue about probabilities, the following considerations may be suggestive : The natural powers of animals-horses, dogs, birds, and others can be improved by training and education, and animals can be taught tricks more or less new to them, but we have no precise information as to any changes of structure associated with these acquirements. Individual animals are sometimes demonstrably affected by use or disuse. Thus Packard cites a few cases in which some animals—usually with normal eyes—have had these affected by disuse and darkness; he instances the variations in the eyes of a Myriapod and an Insect living in partial daylight near the entrance of caves, the change in the eyes of the common Crustacean Gammarus pulex after confinement in darkness, the fact that the eyes of some other Crustaceans in a lake were smaller the deeper the habitat. There are many more or less blind animals, and Packard says no animal or series of generations of animals, wholly or in part, can lose the organs of vision unless there is some appreciable physical cause for it.” If so, it is probable that the appreciable physical cause has been a direct factor in producing the blindness. 66 Not a few young animals have structures, such as eyes and legs, which are not used and soon disappear in adult life. Thus the little crab Pinnotheres, which lives inside bivalves and sea-cucumbers, keeps its eyes until it has established itself within its host. Then they are completely covered over and degenerate. The same is true of many internal parasites, and Semper concludes that "we must refer the loss of sight to disuse of the organ." Perhaps the same is true of some blind cave-animals, in which the eyes are less degenerate in the young, and of the mole, whose embryos have between the eyes and the brain normal optic nerves which usually degenerate in each individual lifetime. The theory that many structures in animals are due to the inherited results of use and disuse has this advantage, that it suggests a primary cause of change, whereas the other theory assumes the occurrence of favourable variations and proceeds to show how they might be accumulated in the course of natural selection, that is to say by a secondary factor in evolution. When we find in a large number of entirely distinct forms that the same habit of life is associated with the same peculiarities, there is a likelihood that the habit is a direct factor in evolving these. Thus sluggish and sedentary animals in many different classes tend to develop skeletons of lime, as in sponges, corals, sedentary worms, lamp-shells, Echinoderms, barnacles, molluscs. Professor Lang has recently made a careful study of sedentary creatures, and this result at least is certain that the same peculiarity often occurs in many different types with little in common except that they are sedentary. But till one can show that sedentary life necessarily involves for instance a skeleton of lime or something equivalent, we are still dealing only with probabilities. 2. The Influence of Surroundings.—In ancient times men saw the threads of their life passing through the hands of three sister-fates-of one who held the distaff, of another who offered flowers, and of a third who bore the abhorred shears of death. In Norseland the young child was visited by three sister Norns, who brought characteristic gifts of past, present, and future, which ruled the life as surely as did the hands of the three Fates. So too in days of scientific illumination, we think of the dread three, but, clothing our thoughts in other words, speak of life as determined by the organism's legacy or inheritance, by force of habit or function, and by the influences of external conditions or environment. What the living organism is to begin with, what it does or does not in the course of its life, and what surrounding influences play upon it,-these are the three Fates, the three Norns, the three Factors of Life. Organism, function, and environment are the sides of the bio |