As regards the latter point, it may be noticed that not in any part of his writings, so far as I can find, does Mr. Wallace allude to the highly important fact of colours in animals being so largely due to these purely physical causes. Everywhere he argues as if colours were universally due to pigments; and in my opinion this unaccountable oversight is the gravest defect in Mr. Wallace's treatment both of the facts and the philosophy of colouration in the animal kingdom. For instance, as regards the particular case of sexual colouration, the oversight has prevented him from perceiving that his theory of "brilliancy" as due to "a surplus of vital energy," is not so much as logically possible in what must constitute at least one good half of the facts to which he applies it--unless he shows that there is some connection between vital energy and the development of striations, imprisonment of air-bubbles, &c. But any such connection--so essentially important for his theory--he does not even attempt to show. Lastly, and quite apart from these remarkable oversights, even if Mr. Tylor's hypothesis were as reasonable and well-sustained as it is fanciful and inadequate, still it could not apply to _sexual_ colouration: it could apply only to colouration as affected by physiological functions common to both sexes. Yet it is in order to furnish a "preferable substitute" for Mr. Darwin's theory of _sexual_ colouration, that Mr. Wallace adduces the hypothesis in question as one of "great weight"! In this matter, therefore, I entirely agree with Poulton and Lloyd Morgan. INDEX A. Accident, Darwin's use of the word, 334-340; beauty due to, 408, 409. Achromatin, 126-134. Acquired characters, _see_ Characters. _Acræa eurita_, 328. Adaptation, facts of, in relation to theory of natural selection, 401-403, 411. Adaptive characters, _see_ Characters. Æsthetic sense in animals, 380-385; _see_ Beautiful. Agassiz, Prof. A., on fauna of the Mammoth cave, 70. Alpine plants, 209, 210, 440-442. _Amauris niavius_, 328. _Amblyornis inornata_, 381-383. _Amphioxus_, 137, 138, 145, 146. Analogy, 38, 50-65, 176, 177, 347-350. Anthropoid, _see_ Apes. Antlers, 98-100, 167-169. Ants, co-operative instincts of, 268; leaf-cutting, 332; keeping aphides, 292. Ape, eye of, 75; _appendix vermiformis_ of, 84-86. Apes, ears of, compared with those of man, 88; muscles of, 77, 82, 83; feet of, 77, 78; tail of, compared with that of man, 82-84; hair of, compared with that of man, 89-91; teeth of, compared with those of man, 92-94; flattening of tibiæ of, 95, 96. Aphides, 292. _Appendix vermiformis_ of man compared with that of orang, 84-86. _Apteryx_, 68, 69. _Archæopteryx_, 171-173. Arctic plants, 209, 210, 440-442. Argyll, Duke of, on natural selection, 334-362. Aristotle, his idea of scientific method, 1; on classification, 23, 24. Arm, distribution of hair on, in man and apes, 89-92. _Arthropoda_, embryology of, 155.