OCR Text |
Show ..... - - - -- -- 296 LAWS OF VARIATION. CliAP. XXIV. inserted the Rpur of a cock into the comb, which is well supplied with blood-vessels, it grew in one case in a spiral direction to a length of six inches, and in another case forward, like a horn, so that the bird could not touch the ground with its beak. But whether M:r. Herbert Spencer's view of the exudation of nutritive matter due to increased movement and pressure, will fully account for the augmented size of bones, ligaments, and especially of internal glands and ne1·ves, seems doubtful. According to the interesting observations of M:. Sedillot, 14 when a portion of one bone of the leg or fore-arm of an animal is removed and is not replaced by growth, the associated bone enlarges till it attains a bulk equal to that of tlle two bones, of which it has to perform the functions. This is best exhibited in dogs in which the tibia bas been removed; the companion bone, which is naturally almost filiform and not one-fifth the size of the other, soon acquires a size equal to or greater than the tibia. Now, it is at first difficult to believe that increased weight acting on a straight bone could, by alternately increased and diminished pressure, cause nutritive matter to exude from the vessels which permeate the periosteum. Nevertheless, the observations adduced by M:r. Spencer, 15 on the strengthening of the bowed bones of rickety children, along their concave sides, leads to the belief that this is possible. Mr. H. Spencer has also shown that the ascent of the sap in trees is aided by the rocking movement caused by the wind · and the sap strengthens the trunk " in proportion to the stres~ " to be borne ; since the more severe and the more repeated the " strains, the greater must be the exudation fi·om the vessels " in~o t~e surrounding tissue, and the greater the thickening of "this tissue by secon.dary d~posits." 16 But woody trunks may be formed of hard tissue without their having been subjected to any movement, as we see with ivy closely attached to old walls. In all these cases, it is very difficult to disentanO'le the effe~ts of long-e?ntinued selection from those consequ~nt on the mcreased ac~wn.or movement of the part. Mr. H. Spencer 17 acknowledges this difficulty, and gives as an instance the spines 1 ~ : Corupt~s ~endus,' Sept. 26th, 1864, p. 539. Io The PrmCiples of Biology,' vol. ii. p. 243. 16 Idem, vol. ii. p. 269. 17 Idem, vol. ii. p. 273. CHAP. XXIV. EFFECTS 01~ USE AND DISUSE. 297 or thorns of trees, and the shells of nuts. Here we have extremely hard woody tissue without the possibility of any movement to cause exudation, and without, as far as we can see, any other directly exciting cause; aud as the hardness of these parts is of manifest service to the plant, we may look at the result as probably due to 1he selection of so-called spontaneous variations. Every one knovvs that hard work thickens the epidermis on the hands; and when we hear that with infants long before their birth the epidermis is thicker on the palms and soles of the feet than on any other part of the body, as was observed with admiration by Albinus,18 we are naturally inclined to attribute this to the inherited effects of long-continued use or pressure. We are tempted to extend the same view even to the hoofs of quadrupeds; but who will pretend to determine how far natural selection may have aided in the formation of structures of such obvious importance to the animal? That use stTengthens the muscles may be seen in the limbs of artisans who follow different trades; and when a muscle is strengthened, the tendons, and the crests of bone to which they are at.tached, become enlarged; and this must likewise be the case with the blood-vessels and nerves. On the other hand, when a limb is not used, as by Eastern fanatics, or when the nerve supplying it with nervous power is effectually destroyed, the muscles wither. So again, when the eye is destroyed the optic nerve becomes atrophied, sometimes even in the course of a few months.19 The Proteus is furnished with branchire as well as with lungs: and Schreibers20 found than when the animal was compelled to live in deep water the branchire were developed to thl·ice their ordinary size, and the lungs were partially atrophied. When, on the other hand, the animal was compelled to live in shallow water, the lungs became larger and more vascular, whilst the branchire disappeared in a more or less complete degree. Such modifications as these are, however, of comparatively little value for us, as we do not actually ]mow that they tend to be inherited. In many cases there is reason to believe that the lessened use of various organs has affected the corresponding parts in the offspring. But there is no good evidence that this ever follows in the comse of a single generation. It appears, as in the case of general or indefinite variability, that several generations must be subjected to changed habits for any appreciable result. Our domestic fowls, ducks, and geese have almost lost, not 1s Pnget, 'Lectures on Pathology,' vol. ii. p. 209. 19 Muller's 'Phys.,' Eng. translat., pp. 54, 791. Prof. Reed bus given ('Physiological and Anat. Re!!earches,' p. 10) a curious account of the atrophy of the limbs of rabbits after the destruction of the nerve. 20 Quoted by Lecoq, in ' Geograph, . Bot.,' tom. i., 1854, p. 182. |