In our work on _The Cave Animals of North America_,[221] after stating that Darwin in his _Origin of Species_ attributed the loss of eyes "wholly to disuse," remarking (p. 142) that after the more or less perfect obliteration of the eyes, "natural selection will often have effected other changes, such as an increase in the length of the antennæ or palpi, as a compensation for blindness," we then summed up as follows the causes of the production of cave faunæ in general: "1. Change in environment from light, even partial, to twilight or total darkness, and involving diminution of food, and compensation for the loss of certain organs by the hypertrophy of others. "2. Disuse of certain organs. "3. Adaptation, enabling the more plastic forms to survive and perpetuate their stock. "4. Isolation, preventing intercrossing with out-of-door forms, thus insuring the permanency of the new varieties, species, or genera. "5. Heredity, operating to secure for the future the permanence of the newly originated forms as long as the physical conditions remain the same. "Natural selection perhaps expresses the total result of the working of these five factors rather than being an efficient cause in itself, or at least constitutes the last term in a series of causes. Hence Lamarckism in a modern form, or as we have termed it, Neolamarckism, seems to us to be nearer the truth than Darwinism proper or natural selection."[222] In an attempt to apply Lamarck's principle of the origin of the spines and horns of caterpillars and other insects as well as other animals to the result of external stimuli,[223] we had not then read what he says on the subject. (See p. 316.) Having, however, been led to examine into the matter, from the views held by recent observers, especially Henslow, and it appearing that Lamarck was substantially correct in supposing that the blood (his "fluids") would flow to parts on the exposed portions of the body and thus cause the origin of horns, on the principle of the saying, "_ubi irritatio, ibi affluxus_," we came to the following conclusions: "The Lamarckian factors (1) change (both direct and indirect) in the _milieu_, (2) need, and (3) habit, and the now generally adopted principle that a change of function induces change in organs,[224] and in some or many cases actually induces the hypertrophy and specialization of what otherwise would be indifferent parts or organs;--these factors are all-important in the evolution of the colors, ornaments, and outgrowths from the cuticle of caterpillars." Our present views as to the relations between the Lamarckian factors and the Darwinian one of natural selection are shown by the following summary at the end of this essay. "1. The more prominent tubercles, and spines or bristles arising from them, are hypertrophied piliferous warts, the warts, with the seta or hair which they bear, being common to all caterpillars.