« AnteriorContinuar »
the feathers, which are shed periodically, enable them to throw off that element without overtaxing their renal organs.
In man the loss of a furry covering to the skin may be accounted for by the same hypothesis. In this case it is scarcely possible to doubt that we see the effect of correlated nutrition. As vast stores of material are needed for the nourishment of the nervous system (so far more highly developed in man than in any other animal), it follows that some other albuminous tissue must have suffered, or the balance of the economy would have failed. As the brain gradually enlarged, the hairy covering of the skin was most easily dispensed with. Phosphorus was likewise required in larger quantities for the nervous system; and the osseous became reduced in size.
The above argument may be briefly summarized as economy of nutrition ; and it will become apparent that economy of nutrition is of all conditions that which conduces most surely to the survival of the race.
A given pabulum being supplied, certain essential structures are nourished, and the residue is economized in the production or modification of other parts, often giving rise to ornamental appendages or bright colours ; whilst the action of the same principle correlates the modifications of different organs or parts with each other.
LAMARCK attributed much to the effect of use and disuse; and his views do not appear to have received the attention of late which they deserve. Great as the effects of natural selection, or Darwin's Law, undoubtedly are, there are many facts to show that Lamarck's Law of variation from use is probably only second to it in its influence on the evolution of complex forms. No doubt many of Lamarck's illustrations are fanciful and unwarranted; but wherever nutrition is affected by activity of function, it is impossible to disbelieve in its wide and extended action.
It is an established physiological fact that the nutrition of an organ is much influenced by its activity. The beautiful researches of Dr. Parkes on the excretion of nitrogen throw much light upon this subject by the manner in which they elucidate the law of nutrition in muscular tissue. It has long been known that use tends to increase the nutrition of an organ, and that disuse determines its waste, or at least materially interferes
with its nutrition. The arm of a blacksmith, with its marvellous muscular development, is a familiar example of the effect of use; whilst the emaciation of the limbs of a patient confined to a bed from an accident is no less familiar as an example of the effect of disuse. It was not, however, until Dr. Parkes showed that the quantity of nitrogen excreted from the body is increased by rest and diminished by muscular exercise, that the truth became known that the growth of muscle is most active during the active performance of function. As nitrogen is only required in the economy for the repair and production of tissue, it follows that, if the nitrogenized tissues retain the same weight, the same quantity of nitrogen that is taken in as food will be again excreted, whether derived directly from the food or from tissue which has been replaced by new material ; if more tissue is developed or built, less nitrogen will be excreted than is taken in; if less tissue is formed than degenerates, then more nitrogen will be excreted than the food supplies. During exercise, less nitrogen is excreted than is derived from the food; hence more is converted into tissue than degenerating tissue gives back. When, however, rest follows exertion, the over-nourished muscles give back some of this extra nitrogen, and hence more is excreted than is afforded by the aliment. The same law probably appertains to
every organ—the greater its activity, the greater its tendency to grow. In this way we may account, to some extent at least, for the proportions of the body. The shape of the bones may, and probably does, become modified in a similar manner : the tendinous insertions of muscles, being subjected to a greater strain, become hypertrophied; bone-cells or osteoblasts originate in the subjacent periosteum under the new stimulus, and a crest of bone is produced. The strong tendency for cartilage and bone to be developed in the large tendons of some animals favours this view; and the following remarkable passage in the writings of Mr. Wallace is evidence of high value upon this point. Writing of the Orang-outang, he says :“When we examine the crania, we find remarkable differences of form, proportion, and dimensions, no two being exactly alike. . . . . This variation enables us to explain satisfactorily the marked difference presented by the single-crested and double-crested skulls, which have been thought to prove the existence of two large species of Orang. The external surface of the skull varies considerably in size, as do also the zygomatic aperture and the temporal muscle ; but they bear no necessary relation to each other, a small muscle often existing with a large cranial surface, and vice versa. Now those skulls which have the largest and strongest
EFFECTS 01 DISUSE.
EFFECTS OF DISUSE. jaws and the widest zygomatic aperture have the muscles so large that they meet on the crown of the skull, and deposit the bony ridge which separates them, and which is the highest in that which has the smallest cranial surface. In those which combine a large surface with comparatively weak jaws and small zygomatic aperture, the muscles on each side do not extend to the crown, a space of from one to two inches remaining between them; and along their margins small ridges are formed. Intermediate forms are found; and the size and form of the ridges are independent of age”*.
Checked and guided by natural selection, the hypertrophy of tissue, directly due to its activity, undoubtedly leads to the most important modifications, especially as the additional supply of nutriment would probably lead to additional variability, a probability already mentioned as Andrew Knight's view.
Mr. Darwin, in his Origin of Species,' has given a long series of facts relating to the loss of organs from disuse. The rudimentary condition of the eyes of creatures living in darkness, as in caves and subterranean burrows, is one of the most important, as we may infer safely that the defective condition in this case is due to want of use, since the eyes are well developed in the embryo and
* Quoted from Huxley's Man's Place in Nature,' p. 40.