“The long neck and the form of the giraffe offer a curious case. We know that the giraffe is the tallest of all animals. It inhabits the centre of Africa, living in those localities where the earth is nearly always dry and without herbage. It is obliged to browse on the foliage of trees, and this leads to its stretching continually upwards. As a result of this habit, carried on for a long time, in all the individuals of the race, the anterior limbs have become longer than the posterior, and its neck has also lengthened, so that the giraffe without rising on its hind-legs stretches up its neck and can reach to the height of six metres.”
The curved claws of the carnivora have arisen from the necessity of grasping their prey. The power of retracting the claws has also been acquired by the effort to draw them in when running over hard ground. The abdominal pouch of the kangaroo, in which the young are carried, opens anteriorly, and this has led to the animal standing erect so that its young are not injured. In consequence, the fore-legs have become shorter through disuse, and the hind-legs have become stronger through use. The tail, which is also used as a support, has become enormously thick at its base.
The sloth has been compelled to seek refuge in the trees, and has taken up its abode permanently there, feeding on leaves. Its movements are limited to those involved in crawling along the limbs in order to reach the leaves. After feeding it remains inactive and sluggish, these habits being provoked by the heat of the climate. The results of its mode of life have been to cause the arms to become elongated due to the habit of the sloth of grasping the limbs of the tree; the claws of the fingers and toes have also become long and hooked in order to retain their hold. The digits that do not make any individual movements have lost the power to do so, and have become fused, and can only be bent in and straightened out. The thighs, being bent out to clasp the larger branches, have caused the pelvis to widen, and, in consequence, the cotyloid cavities have become directed backward. Many of the bones of the skeleton have become fused, as a result of the immobility of the animal.
Lamarck says, that “Nature, in producing, successively, all the species of animals, beginning with the most imperfect, or the most simple, and terminating with the most perfect, has gradually complicated their organization. These animals becoming scattered throughout the habitable regions of the globe each species has received from the influences of its surroundings its present habits, and the modifications of the parts the use of which we recognize.”
Such are Lamarck’s views and a fairly complete statement of the facts from which he draws his conclusions. His illustrations appear naïve, and often not a little ludicrous, but it must be admitted that, despite their absurdities, his theory appears in some cases to account wonderfully well for the facts. The long legs of wading birds, the long neck and disproportionately long fore-legs of the giraffe, the structure of the sloth, and particularly the degeneration of the eyes of animals living in the dark, seem to find a simple explanation in the principle of the inheritance of acquired characters. But the crucial point of the entire theory is passed over in silence, or rather is taken for granted by Lamarck, namely, the inheritance in the offspring of the characters acquired through use or disuse in the parent. He does not even discuss this topic, but in several places states unreservedly that the increase or decrease of a part reappears in the next generation. It is here that Lamarck’s theory has been attacked in more modern times, for as soon as experimental proof was demanded to show that the results of use or of disuse of an organ is inherited, no such proof was forthcoming. Yet the theory is one that has the great merit of being capable of experimental test, and it is astonishing to find that, with the immense amount that has been written by his followers, so few attempts have been made to give the theory a thorough test. The few results that have been obtained are not, however, favorable to the theory, but almost the only attempts at experiment that have been made in this direction have been those of mutilating certain parts; and were it not for popular belief to the effect that such mutilations are inherited, one would least expect to get evidence for or against the theory in this direction. Lamarck himself believed that the changes were slowly acquired, and I think modern Lamarckians are justified in claiming that the validity of the theory can only be tested by experiments in which the organism is subjected to influences extending over a considerable period, although Lamarck appears to have believed that the first results may appear quite soon. Before expressing any opinion in regard to the probability of the theory, let us examine what the followers of Lamarck have contributed in the way of evidence to the theory, rather than the applications that they have made of the theory. We shall also find it profitable to consider some of the modern criticism, to which the theory has been subjected.
Despite the contempt with which Darwin referred to Lamarck’s theory, he himself, as we have seen, often made use of the principle of the inheritance of acquired characters, and even employed the same illustrations cited by Lamarck. Darwin seems to have misunderstood Lamarck’s view, and to have accepted the current opinion that Lamarck supposed an animal acquired a new organ by desiring or needing it. Darwin says, “Heaven forefend me from Lamarck’s nonsense of a tendency to progressive adaptation from the slow willing of the animals.” Darwin speaks of Lamarck as stating that animals will that the egg shall be a particular form so as to become attached to particular objects. Lamarck’s latest biographer, Packard, says he is unable to find any statements of this sort in Lamarck’s writings.
The following cases that Darwin tried to explain through the inheritance of acquired characters are exactly like those to which Lamarck applied his theory. The bones of the wing of the domestic duck weigh less than those of the wild duck, and the bones of the leg more. Darwin believes this is due to the effects of the inheritance of acquired characters. The drooping ears of many domestic mammals are also explained by him as a result of disuse—“the animals being seldom much alarmed.” In speaking of the male of the beetle, Onites apelles, Darwin quotes Kirby to the effect that the tarsi are so habitually lost that the species has been described without this part of the foot. In the sacred beetle of Egypt the tarsus is totally absent. Hence he concludes that the absence of tarsi in the sacred beetle, and the rudimentary condition of the tarsus in others, is probably the result of disuse, rather than a case of inheritance of a mutilation. Darwin grants that “the evidence that accidental mutilations can be inherited is at present not decisive, but the remarkable case observed by Brown-Séquard in guinea-pigs of the inherited effects of operations should make us cautious in denying this tendency.”
The wingless condition of several insects inhabiting oceanic islands has come about, Darwin thinks, through disuse. The ostrich also, owing to its increase in size, made less use of its wings and more use of its legs, with the result that its wings degenerated and its legs got stronger. The rudimentary condition of the eyes of the mole is the result of disuse, “aided perhaps by natural selection.” Many of the animals inhabiting the caves of Kentucky and of Carniola are blind, and this is ascribed to disuse. “As it is difficult to imagine that the eyes, though useless, could be in any way injurious to animals living in darkness, their loss may be attributed to disuse.” The long neck of the giraffe Darwin attributes partly to natural selection and partly to use.
These references will suffice to show that Darwin is in full accord with the main argument of Lamarck. In fact, the curious hypothesis of pangenesis that Darwin advanced was invented partly to account for the inheritance of acquired characters. Despite the hesitancy that Darwin himself felt in advancing this view, and contrary to Huxley’s advice, he at last published his provisional hypothesis of pangenesis in the twenty-seventh chapter of his “Animals and Plants under Domestication.”