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deeply affect the lives and happiness of the great bulk of the people. It is a great satisfaction that his last letter to me, written within nine months of his death, and terminating a correspondence which had extended over a quarter of a century, should be so cordial, so sympathetic, and broadminded.
In 1870 he had written to me, “I hope it is a satisfaction to you to reflect-and very few things in my life have been more satisfactory to me—that we have never felt any jealousy towards each other, though in some sense rivals. I believe I can say this of myself with truth, and I am absolutely sure that it is true of you." The above long letter will show that this friendly feeling was retained by him to the last, and to have thus inspired and retained it, notwithstanding our many differences of opinion, I feel to be one of the greatest honours of my life. I have myself given an estimate of Darwin's work in my “Debt of Science to Darwin," published in my “Natural Selection and Tropical Nature," in 1891. But I cannot here refrain from quoting a passage from Huxley's striking obituary notice in Nature, summing up his work in a single short paragraph: “None have fought better, and none have been more fortunate than Charles Darwin. He found a great truth, trodden underfoot, reviled by bigots, and ridiculed by all the world; he lived long enough to see it, chiefly by his own efforts, irrefragably established in science, inseparably incorporated with the common thoughts of men, and only hated and feared by those who would revile but dare not. What shall a man desire more than this ?"
The Chief Differences of Opinion between Darwin and myself.--As this subject is often referred to by objectors to the theory of natural selection, and it is sometimes stated that I have myself given up the most essential parts of that theory, I think it will be advisable to give a short statement of what those differences really are, and how they affect the theory in question. Our only important differences were on four subjects, which may be considered separately.
1. The Origin of Man as an Intellectual and Moral
Being:-On this great problem the belief and teaching of Darwin was, that man's whole nature-physical, mental, intellectual, and moral-was developed from the lower animals by means of the same laws of variation and survival; and, as a consequence of this belief, that there was no difference in kind between man's nature and animal nature, but only one of degree. My view, on the other hand, was, and is, that there is a difference in kind, intellectually and morally, between man and other animals; and that while his body was undoubtedly developed by the continuous modification of some ancestral animal form, some different agency, analogous to that which first produced organic life, and then originated consciousness, came into play in order to develop the higher intellectual and spiritual nature of man. This view was first intimated in the last sentence of my paper on the "Development of Human Races under Natural Selection," in 1864, and more fully treated in the last chapter of my " Essays," in 1870.
These views caused much distress of mind to Darwin, but, as I have shown, they do not in the least affect the general doctrine of natural selection. It might be as well urged that because man has produced the pouter-pigeon, the bull-dog, and the dray-horse, none of which could have been produced by natural selection alone, therefore the agency of natural selection is weakened or disproved. Neither, I urge, is it weakened or disproved if my theory of the origin of man is the true one.
2. Sexual Selection through Female Choice.-Darwin's theory of sexual selection consists of two quite distinct parts-the combats of males so common among polygamous mammals and birds, and the choice of more musical or more ornamental male birds by the females. The first is an observed fact, and the development of weapons such as horns, canine teeth, spurs, etc., is a result of natural selection acting through such combats. The second is an inference from the observed facts of the display of the male plumage or ornaments ; but the statement that ornaments have been developed by the female's choice of the most beautiful male
because he is the most beautiful, is an inference supported by singularly little evidence. The first kind of sexual selection I hold as strongly and as thoroughly as Darwin himself; the latter I at first accepted, following Darwin's conclusions from what appeared to be strong evidence explicable in no other way; but I soon came to doubt the possibility of such an explanation, at first from considering the fact that in butterflies sexual differences are as strongly marked as in birds, and it was to me impossible to accept female choice in their case, while, as the whole question of colour came to be better understood, I saw equally valid reasons for its total rejection even in birds and mammalia.
But here my view really extends the influence of natural selection, because I show in how many unsuspected ways colour and marking is of use to its possessor. I first stated my objections to "female choice” in my review of the “ Descent of Man" (1871), and more fully developed it in my
Tropical Nature” (1878), while in my “Darwinism” (1889), I again discussed the whole subject, giving the results of more mature consideration. I had, however, already discussed the matter at some length with Darwin, and in a letter of September 18, 1869, I gave him my general argument as follows :
"I have a general and a special argument to submit.
“1. Female birds and insects are usually exposed to more danger than the male, and in the case of insects their existence is necessary for a longer period. They therefore require, in some way or other, an increased amount of protection.
“ 2. If the male and female were distinct species, with different habits and organizations, you would, I think, admit that a difference of colour, serving to make that one less conspicuous which evidently required more protection than the other, had been acquired by natural selection.
“ 3. But you admit that variations appearing in one sex are (sometimes) transmitted to that sex only. There is, therefore, nothing to prevent natural selection acting on the two sexes as if they were two species.
“4. Your objection that the same protection would, to a
certain extent, be useful to the male seems to me quite unsound, and directly opposed to your own doctrine so convincingly urged in the Origin,' that natural selection never improves an animal beyond its needs. Admitting, therefore, abundant variation of colour in both sexes, it is impossible that the male can be brought by natural selection to resemble the female (unless such variations are always transmitted), because the difference in their colours is for the purpose of making up for their different organization and habits, and natural selection cannot give to the male more protection than he requires, which is less than in the female.
“5. The striking fact that in all protected groups the females usually resemble the males (or are equally brightly coloured) shows that the usual tendency is to transmit colour to both sexes when it is not injurious to either.
“Now for the special argument.
“6. In the very weak-flying Leptalis both sexes mimic Heliconidæ. But in the much stronger-flying Papilio, Pieris, and Diadema, it is the female only that mimics the protected group, and in these cases the females often acquire brighter and more conspicuous colours than the male.
“7. No case is known of a male Papilio, Pieris, or Diadema, alone, mimicking a protected species; yet colour is more frequent in males, and variations are always ready for the purpose of sexual or other forms of selection.
“8. The fair inference seems to be that each species, and also each sex, can only be modified by selection just as far as is absolutely necessary—not a step further. A male, being by structure and habits less exposed to danger, and therefore requiring less protection than the female, cannot have an equal amount of protection given to it by natural selection ; but the female must have some extra protection to balance her greater exposure to danger, and she rapidly acquires it in one way or another.
"9. The objection as to male fish, which seem to require protection, yet have sometimes bright colours, seems to me of no more weight than is the existence of some unprotected
species of white Leptalis as a disproof of Bates' theory of mimicry, or that only a few species of butterfly resemble leaves,—or that the habits and instincts that protect one animal are absent in allied species. These are all illustrations of the many and varied ways in which nature works to give the exact amount of protection it needs to each species."
3. Arctic Plants in the Southern Hemisphere, and on Isolated Mountain-tops within the Tropics.—Having paid great attention to the whole question of the distribution of organisms, I was obliged to reject Mr. Darwin's explanation of the above phenomena by a cooling of the tropical lowlands of the whole earth during the glacial period to such an extent as to allow large numbers of north-temperate and Arctic plants to spread across the continents to the southern hemisphere, and, as the cold passed away, to ascend to the summits of isolated tropical mountains. The study of the floras of oceanic islands having led me to the conclusion that the greater part of their flora was derived by aërial transmission of seeds, either by birds or by gales and storms, I extended this view to the transmission along mountain ranges, and from mountain-top to mountain-top, as being most accordant with the facts at our disposal. I explained my views at some length in “ Island Life," and later, with additional facts, in “Darwinism."
The difficulties in the way of Darwin's view are twofold. First, that a lowering of temperature of inter-tropical lowlands to the required extent would inevitably have destroyed much of the overwhelming luxuriances and variety of plant, insect, and bird life that characterize those regions. This has so impressed myself, Bates, and others familiar with the tropics as to render the idea wholly inconceivable ; and the only reason why Darwin did not feel this appears to be that he really knew nothing personally of the tropics beyond a few days at Bahia and Rio, and could have had no conception of its wonderfully rich and highly specialized fauna and flora. In the second place, even if a sufficient lowering of temperature had occurred during the ice-age, it would not account for