THERE is a slight difference between all three of the answers to Mr. Murphy's queries on Protective Mimicry (vol. xiv. pp. 309, 329); but, I think, the authors of those replies are unanimously over-hasty to call in the aid of protective selection. I cannot but think that the perpetuation of nascent variations may more safely be attributed to causes identical with those in which the variations originate. If this be so, though the origin of variation is necessarily beyond the scope of any selection-theory, these causes continuing to act on the varying organism become of vasl importance to the evolutionist. Too much importance cannot, I think, be given in this connection, to the principle of economy of nutrition, or balancement of growth, formulated by Aristotle, in the words “άμα δε την αντην νπεροχην εισ πολλονσ τοπονσ αδνυατει διανμεμειν η πνσισ,” and by Goethe, in the expression “Nature, in order to spend on one side, is forced to economise on the other.” I gave one example of this law in a previous letter (NATURE, vol. xiii. p. 107) in speaking of the indirect uses of the waste, or secondary products of metastasis in plants; but as the subject is admirably sketched in Mr. Lowne's suggestive essay on “The Philosophy of Evolution,” a work too little known or appreciated, I will give here an outline of the argument. Food may be divided into three parts—for nutrition, for the production of energy and waste, or excretion. In many lower organisms the excreted material forms a simple shell; in plants manna, nectar, and resins belong to this group. The chief form of energy in the organic kingdom is that resulting from the oxidation of carbon, chiefly characteristic of animals, while plants secrete the energy-producing material. In higher organisms it is physiologically advantageous that the parts of an organism should differ in the kind of nourishment they require and thus act, as Sir James Paget has shown, as excretory organs to one another. Thus all animals which feed on large quantities of comparatively slightly nutritious matter have a complicated digestive apparatus, and a strong tendency to the production of large skeletons or cutaneous organs, which relieve the special excretory organs. The stag applies a large portion of the calcareous salts derived from the herbage to the production of horns in the male and the bones of the young in the female. The thoracic appendages of the lamellicorns and the beaks of the toucan and horn-bill are given as further examples, and “the dermal appendages of reptiles and the feathers of birds, rich in pigment and nitrogen are probably entirely excrementitious to the other tissues.” Mr. Lowne makes an interesting final application of this hypothesis to the loss of the hairy covering of the human skin, it being the albuminous tissue most easily dispensed with to nourish the highly developed nervous system. “Phosphorus was likewise required in large quantities and the osseous system became reduced in size.” The composition of the nutrient fluid of the organism remaining constant, the excreted matter will be so also, and thus, for example, a rudimentary horn or a pigment may be produced by a change of food and preserved, while the food remains the same, by a physiological selection, as preventing the overtaxing of the kidneys, before sexual selection or protective selection come into play. I have an instance in point before me. Two plants of variegated kale under the influence of the late drought have produced axial structures from the midribs and veins of all their leaves, and I have no doubt that were their seedlings grown under similar nutritive conditions, a race of plants thus substituting fibro-vascular tissue for the usually abnormal development of parenchyma in the kale would be produced.
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