NATURAL SELECTION NOT THE EXCLUSIVE :MEANS OF MODIFICATION. it should he observed that Darwin frankly expressed the eonviction that natural selection, though the most important, has not been the exclusive means of modification. He allowed that the absenee of eyes in cave ani mals is not the result of natural selection, say ing: attribute their loss wholly to disuse." THE CAUSES OF VARIATION. In the first edi tion of his Variation of Animals and Plants Under Domestication, Darwin. after ning the definite action of the environment. added: "A new sub-variety would thus lie produced without the aid of natural selection," but this passage was omitted from the seeond edi tion. Besides the Origin of Species Darwin puh lished a number of other works, the most impor tant of which was The Variation of _Inimals and Plants under Domestication (1868). Toward the end of his life he gave more attention to the causes of variation, which at first he said were unknown. and in the work just cited he says: "Changes of any kind in the conditions of life, even extremely slight change-s, often suffice to cause variability" (2 ed., ii., p. 2.55) ; and again: "Variations of all kinds and degrees are directly or indirectly caused by the conditions of life to which each being, and more especially its an cestors, have been exposed," adding: "To put the ease under another point of view, if it were possible to expose all the individuals of a species during many to absolutely uniform of life, there would he no pp. 253, 255-256). ile attributes the dif
ferences in races of cats living in Paraguay, at Alombasa, East Africa, and in Antigua. "to the direct action of difrerent conditions of life" (i.. p. 49) ; so with the horses of the South American pampas and of Puna. Ile refers to 1)r. .1. A. Allen's conclusions relative to the direct action of the climate in producing geographical varieties of birds, and concludes that "these differences must be attributed to the direct action of tem peratth•e" (ii., p. 271). SO also. accepting the results of Meehan's couipcc•isoos on the leaves of 20 kinds of American trees with their nearest European allies, Darwin candidly admits that "such difference cannot have been gained through natural selection, and must be attributed to the long continued action of a different cli mate" (ii., p. 271).
The objections to his theory raised by Darwin himself he discusses with his usual candor. Of these the most important is the absenee, to use his OWn words, of the "interminable number of in termediate forms," which must have existed, "linking together all the species in each group by gradations as fine as are our present va rieties." lie says he can only answer this objec tion "on the supposition that the geological record is far more imperfeet than most geologists believe." The other objection is the existence of two or three castes of worker or sterile ants in the same community.