LETTER VII. THE mass and weight of evidence that may be advanced in favour of Evolution is so overwhelming that, for my part, I am unable to look upon the theory as any longer debatable. As regards natural selection, there are some impediments in its way, at least to its entire acceptance, that demand attention; all the more because both adherents and opponents are apt to think, the doctrine has a special bearing upon the argument from design. The inquiry will best be pursued by meeting the objections in the following order: (1) The absence of transitional form; (2) the limits of variability; (3) the time required; (4) the efficiency of natural selection. It is admitted on all hands, even by the most irreconcilable of Darwin's adversaries, that his fairness in dealing with difficulties, nay, his care to seek them, commands respect for the founder of the theory, whatever may be thought of the perfervour of some of his disciples. It has often been said that, in order to fully estimate the weak points of natural selection, we must go to Darwin himself, who, surpassing all others in the knowledge of his subject, is likewise the most painstaking in exposing its defects. The sixth and seventh chapters of the last edition of the "Origin of Species" are devoted to objections which have occurred to its author, and also to such as, up to the present time, have been advanced by others. Amongst the foremost of these at home is Mr. Mivart, to whose strictures, in the "Genesis of Species," Darwin gives his special attention. Some of the arguments in Mr. Murphy's "Habit and Intelligence," and in Mr. Wallace's "Natural Selection," are also well worthy of consideration. But perhaps the ablest attack yet made within equal limits is Professor Tait's skilful essay in the "North British Review" of March 1867. As this is not very accessible, I shall refer to it repeatedly. It is a maxim with Darwin that "Natura non facit saltum." "Natural selection," he tells us, "acts only by taking advantage of slight successive variations. She can never take a great and sudden leap, but must advance by short and sure, though slow steps."1 "This canon, if we look to the present inhabitants alone of the world, is not strictly correct; but if we include all those of past times, whether known or unknown, it must on this theory be strictly true." 2 In the closing chapter of the "Origin of Species" he again repeats the canon, under the conviction that "every fresh addition to our knowledge tends to confirm it." We shall be reminded of these sentences by and by. At present they preface the remark that, "by this theory innumerable transitional forms must have existed; why do we not find them embedded in countless numbers in the crust of the earth?" This is the oldest and most obvious of the objections to natural selection. It does not touch the doctrine of evolution, for evolution. does not pledge itself to minute and gradual changes; but with anti-Darwinians, it is an ultima ratio. "It is not possible," says Mr. Mivart, "to deny the a priori probability of the preservation of at least a few minutely transitional forms in some instances, if every species without exception has arisen exclusively by such minute and gradual transitions."4 Professor Tait asks how it is that, we find such vast numbers of perfectly similar specimens -so many fossils of one variety-and fail to meet with more varieties, which probably far exceeded the number of individuals that have existed of any one variety. "It is very strange that the specimens should be so exactly alike. 1 Origin of Species, p. 156. 2 Ibid., 3 Ibid., p. 414. 4 The Genesis of Species, p. 153. P. 166. "2 as they are, if, in fact, they came and vanished by a gradual change." Darwin himself declares that "the number of intermediate and transitional links between all living and extinct species must have been inconceivably great." In certain formations whole groups of species suddenly make their appearance. What is the answer to this? Mr. Mivart thinks it "absolutely incredible that birds, bats, and pterodactyles should have left the remains they have, and yet not a single relic be preserved in any one instance of any of these different forms of wing in their incipient and relatively imperfect functional condition." Bats have always "presented the exact type of existing forms." "The pterodactyles, again, though a numerous group, are all true and perfect pterodactyles." The geographical distribution of animals strongly favours the Darwinian theory of descent. The resemblance between extinct and existing forms in different parts of the world is otherwise quite inexplicable. Yet Mr. Mivart produces a long list of genera, species of which are met with at most unaccountable distances apart. Families of fishes, for instance, are represented in the fresh waters of India, Java, Borneo, and Aleppo, and again on the West Coast of Africa. "The characinidae (a family of the physostomous fishes) are found in Africa and South America," &c. "Pleurodont iguanian lizards abound in the South American region, but nowhere else, &c. Yet pleurodont lizards, strange to say, are found in Madagascar." Amongst the insectivorous beasts of Madagascar there is one member of the group, the solenodon, "which is also found in Cuba and Hayti." "The connection, however, between the West Indies and Madagascar must surely have been at a time when the great lemurine group was absent," &c.3 Darwin quotes Professor Dana to the effect that, "it is certainly a wonderful fact that New Zealand should have a closer resemblance in its crustacea 2 Origin of Species, p. 266. Ibid., p. 168. 1 Genesis of Species, p. 317; to Great Britain, its antipode, than to any other part of the world." 1 The main reply to these and all similar objections is— the imperfection of the geological record. There is also something to be said about the supposed multitude of the individuals of intermediate forins. Let us hear Darwin on this head before I select a few passages which justify his reliance on the other and greater plea. His opinion is that, we have no right to expect to discover as many varieties as individuals of one variety, because the connecting links would never have constituted large masses. "If we take a varying species inhabiting a very large area, we shall have to adapt two varieties to two large areas, and a third variety to a narrow intermediate zone. The intermediate variety, consequently, will exist in lesser numbers from inhabiting a narrow and lesser area," &c.2 Darwin has found this to be practically the case; hence he concludes "that varieties linking two other varieties together generally have existed in lesser numbers than the forms which they connect," &c. Another matter which objectors are apt to overlook is, "that the parent form of any two or more species would not be in all its characters directly intermediate between its modified offspring, any more than the rock-pigeon is directly intermediate in crop and tail between its descendants, the pouter and fantail pigeons."3 If we want the intermediates between distinct groups of animals, we must seek them in the remote past; and the imperfection of the geological record forbids us to expect any such discovery. It may here be noticed by the way that, uninformed critics point with sceptical scorn to the impassable gulf even between animals low down in the scale; as if the lowest of the vertebrates, or at all events the lancelet, ought, upon the Darwinian theory, to exhibit signs of relationship to the higher forms of molluscs, or to some of the articulated Origin, &c., p. 338. 2 Ibid., p. 136. All references are to the 6th edition, unless otherwise stated. 3 Ibid., p. 408. animals. But it is a mistake, we are told, to suppose that the soft-bodied sea-squirt is descended from any of the soft-bodied molluscs. The entire vertebrate tribe, to which the sea-squirt, and probably the lancelet, belong, has developed from the worm tribe. Consequently, we vertebrates have nothing to do with insects, any further than through our common descent from the protozoa. "Unfortunately," says Professor Häckel, with unfeigned sorrow, "we lose by this relationship, which might otherwise connect us with termites, ants, bees, and other virtuous members of the articulate class."2 To turn now to the imperfection of the geological record. One is embarrassed with the wealth of instances. To begin with, the sweeping announcement may be made (no geologist disputes it), that, "every spot which is now dry land has been sea at some former period, and every part of the space now covered by the deepest ocean has been land."3 This is no guesswork, mind; we have positive proof as to the submersion. The Alps, the Andes, and the Himalaya, give up their marine shells at the respective heights of 8000, 14,000, and 16,000 feet. Even so late as the beginning of the Eocene period, the Alps rose in some places 10,000, and the Pyrenees 11,000 feet. Professor Ramsay found near Snowdon, at 1400 feet above the sea, shells that prove the submersion of Wales so recently as the glacial epoch. But after this the bed of the glacial sea was again laid dry. Great Britain was united, as before, to the Continent; and then came the present state of things. The supposition regarding the bottom of the present ocean is based on the assumption that the proportion of land to water was always about the same. And if the average depth of the sea be roughly taken at admit that Chimborazo with its 21,000 and Everest with its 29,000 feet have been under water, it is easy to believe that the whole bed of the ocean may have been exposed. 15,000 feet, and we 1 Häckel, Evolution of Man, vol. i. p. 251. 2 Ibid. |