"Le cours ordinaire de la nature vivante, est en general toujours constant, toujours le meme; son mouvement, toujours regulier, roule sur deux points inebranlables: l"un, la fecondite sans bornes donnee a toutes les especes; l"autre, les obstacles sans nombre qui reduisent cette fecondite a une mesure determinee et ne laissent en tout temps qu"a peu pres la meme quant.i.te d"individus de chaque espece"... "Les especes les moins parfaites, les plus delicates, les plus pesantes, les moins agissantes, les moins armees, etc., ont deja disparu ou disparaitront."
Erasmus Darwin (See Ernst Krause and Charles Darwin, "Erasmus Darwin", London, 1879.) had a firm grip of the "idea of the gradual formation and improvement of the Animal world," and he had his theory of the process.
No sentence is more characteristic than this: "All animals undergo transformations which are in part produced by their own exertions, in response to pleasures and pains, and many of these acquired forms or propensities are transmitted to their posterity." This is Lamarckism before Lamarck, as his grandson pointed out. His central idea is that wants stimulate efforts and that these result in improvements, which subsequent generations make better still. He realised something of the struggle for existence and even pointed out that this advantageously checks the rapid multiplication. "As Dr Krause points out, Darwin just misses the connection between this struggle and the Survival of the Fittest." (Osborn op. cit. page 142.)
Lamarck (1744-1829) (See E. Perrier "La Philosophie Zoologique avant Darwin", Paris, 1884; A. de Quatref.a.ges, "Darwin et ses Precurseurs Francais", Paris, 1870; Packard op. cit.; also Claus, "Lamarck als Begrunder der Descendenzlehre", Wien, 1888; Haeckel, "Natural History of Creation", English translation London, 1879; Lang "Zur Charakteristik der Forschungswege von Lamarck und Darwin", Jena, 1889.) seems to have thought out his theory of evolution without any knowledge of Erasmus Darwin"s which it closely resembled. The central idea of his theory was the c.u.mulative inheritance of functional modifications. "Changes in environment bring about changes in the habits of animals. Changes in their wants necessarily bring about parallel changes in their habits. If new wants become constant or very lasting, they form new habits, the new habits involve the use of new parts, or a different use of old parts, which results finally in the production of new organs and the modification of old ones." He differed from Buffon in not attaching importance, as far as animals are concerned, to the direct influence of the environment, "for environment can effect no direct change whatever upon the organisation of animals," but in regard to plants he agreed with Buffon that external conditions directly moulded them.
Trevira.n.u.s (1776-1837) (See Huxley"s article "Evolution in Biology", "Encyclopaedia Britannica" (9th edit.), 1878, pages 744-751, and Sully"s article, "Evolution in Philosophy", ibid. pages 751-772.), whom Huxley ranked beside Lamarck, was on the whole Buffonian, attaching chief importance to the influence of a changeful environment both in modifying and in eliminating, but he was also Goethian, for instance in his idea that species like individuals pa.s.s through periods of growth, full bloom, and decline. "Thus, it is not only the great catastrophes of Nature which have caused extinction, but the completion of cycles of existence, out of which new cycles have begun." A characteristic sentence is quoted by Prof. Osborn: "In every living being there exists a capability of an endless variety of form-a.s.sumption; each possesses the power to adapt its organisation to the changes of the outer world, and it is this power, put into action by the change of the universe, that has raised the simple zoophytes of the primitive world to continually higher stages of organisation, and has introduced a countless variety of species into animate Nature."
Goethe (1749-1832) (See Haeckel, "Die Naturanschauung von Darwin, Goethe und Lamarck", Jena, 1882.), who knew Buffon"s work but not Lamarck"s, is peculiarly interesting as one of the first to use the evolution-idea as a guiding hypothesis, e.g. in the interpretation of vestigial structures in man, and to realise that organisms express an attempt to make a compromise between specific inertia and individual change. He gave the finest expression that science has yet known--if it has known it--of the kernel-idea of what is called "bathmism," the idea of an "inherent growth-force"--and at the same time he held that "the way of life powerfully reacts upon all form" and that the orderly growth of form "yields to change from externally acting causes."
Besides Buffon, Erasmus Darwin, Lamarck, Trevira.n.u.s, and Goethe, there were other "pioneers of evolution," whose views have been often discussed and appraised. Etienne Geoffroy Saint-Hilaire (1772-1844), whose work Goethe so much admired, was on the whole Buffonian, emphasising the direct action of the changeful milieu. "Species vary with their environment, and existing species have descended by modification from earlier and somewhat simpler species." He had a glimpse of the selection idea, and believed in mutations or sudden leaps--induced in the embryonic condition by external influences. The complete history of evolution-theories will include many instances of guesses at truth which were afterwards substantiated, thus the geographer von Buch (1773-1853) detected the importance of the Isolation factor on which Wagner, Romanes, Gulick and others have laid great stress, but we must content ourselves with recalling one other pioneer, the author of the "Vestiges of Creation" (1844), a work which pa.s.sed through ten editions in nine years and certainly helped to harrow the soil for Darwin"s sowing. As Darwin said, "it did excellent service in this country in calling attention to the subject, in removing prejudice, and in thus preparing the ground for the reception of a.n.a.logous views."
("Origin of Species" (6th edition), page xvii.) Its author, Robert Chambers (1802-1871) was in part a Buffonian--maintaining that environment moulded organisms adaptively, and in part a Goethian--believing in an inherent progressive impulse which lifted organisms from one grade of organisation to another.
AS REGARDS NATURAL SELECTION.
The only thinker to whom Darwin was directly indebted, so far as the theory of Natural Selection is concerned, was Malthus, and we may once more quote the well-known pa.s.sage in the Autobiography: "In October, 1838, that is, fifteen months after I had begun my systematic enquiry, I happened to read for amus.e.m.e.nt "Malthus on Population", and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circ.u.mstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species."
("The Life and Letters of Charles Darwin", Vol. 1. page 83. London, 1887.)
Although Malthus gives no adumbration of the idea of Natural Selection in his exposition of the eliminative processes which go on in mankind, the suggestive value of his essay is undeniable, as is strikingly borne out by the fact that it gave to Alfred Russel Wallace also "the long-sought clue to the effective agent in the evolution of organic species." (A.R. Wallace, "My Life, A Record of Events and Opinions", London, 1905, Vol. 1. page 232.) One day in Ternate when he was resting between fits of fever, something brought to his recollection the work of Malthus which he had read twelve years before. "I thought of his clear exposition of "the positive checks to increase"--disease, accidents, war, and famine--which keep down the population of savage races to so much lower an average than that of more civilized peoples. It then occurred to me that these causes or their equivalents are continually acting in the case of animals also; and as animals usually breed much more rapidly than does mankind, the destruction every year from these causes must be enormous in order to keep down the numbers of each species, since they evidently do not increase regularly from year to year, as otherwise the world would long ago have been densely crowded with those that breed most quickly. Vaguely thinking over the enormous and constant destruction which this implied, it occurred to me to ask the question, Why do some die and some live? And the answer was clearly, that on the whole the best fitted live. From the effects of disease the most healthy escaped; from enemies the strongest, the swiftest, or the most cunning; from famine the best hunters or those with the best digestion; and so on. Then it suddenly flashed upon me that this self-acting process would necessarily IMPROVE THE RACE, because in every generation the inferior would inevitably be killed off and the superior would remain--that is, THE FITTEST WOULD SURVIVE." (Ibid. Vol. 1. page 361.) We need not apologise for this long quotation, it is a tribute to Darwin"s magnanimous colleague, the Nestor of the evolutionist camp,--and it probably indicates the line of thought which Darwin himself followed. It is interesting also to recall the fact that in 1852, when Herbert Spencer wrote his famous "Leader" article on "The Development Hypothesis" in which he argued powerfully for the thesis that the whole animate world is the result of an age-long process of natural transformation, he wrote for "The Westminster Review" another important essay, "A Theory of Population deduced from the General Law of Animal Fertility", towards the close of which he came within an ace of recognising that the struggle for existence was a factor in organic evolution. At a time when pressure of population was practically interesting men"s minds, Darwin, Wallace, and Spencer were being independently led from a social problem to a biological theory. There could be no better ill.u.s.tration, as Prof. Patrick Geddes has pointed out, of the Comtian thesis that science is a "social phenomenon."
Therefore, as far more important than any further ferreting out of vague hints of Natural Selection in books which Darwin never read, we would indicate by a quotation the view that the central idea in Darwinism is correlated with contemporary social evolution. "The subst.i.tution of Darwin for Paley as the chief interpreter of the order of nature is currently regarded as the displacement of an anthropomorphic view by a purely scientific one: a little reflection, however, will show that what has actually happened has been merely the replacement of the anthropomorphism of the eighteenth century by that of the nineteenth.
For the place vacated by Paley"s theological and metaphysical explanation has simply been occupied by that suggested to Darwin and Wallace by Malthus in terms of the prevalent severity of industrial compet.i.tion, and those phenomena of the struggle for existence which the light of contemporary economic theory has enabled us to discern, have thus come to be temporarily exalted into a complete explanation of organic progress." (P. Geddes, article "Biology", "Chambers"s Encyclopaedia".) It goes without saying that the idea suggested by Malthus was developed by Darwin into a biological theory which was then painstakingly verified by being used as an interpretative formula, and that the validity of a theory so established is not affected by what suggested it, but the practical question which this line of thought raises in the mind is this: if Biology did thus borrow with such splendid results from social theory, why should we not more deliberately repeat the experiment?
Darwin was characteristically frank and generous in admitting that the principle of Natural Selection had been independently recognised by Dr W.C. Wells in 1813 and by Mr Patrick Matthew in 1831, but he had no knowledge of these antic.i.p.ations when he published the first edition of "The Origin of Species". Wells, whose "Essay on Dew" is still remembered, read in 1813 before the Royal Society a short paper ent.i.tled "An account of a White Female, part of whose skin resembles that of a Negro" (published in 1818). In this communication, as Darwin said, "he observes, firstly, that all animals tend to vary in some degree, and, secondly, that agriculturists improve their domesticated animals by selection; and then, he adds, but what is done in this latter case "by art, seems to be done with equal efficacy, though more slowly, by nature, in the formation of varieties of mankind, fitted for the country which they inhabit."" ("Origin of Species" (6th edition) page xv.) Thus Wells had the clear idea of survival dependent upon a favourable variation, but he makes no more use of the idea and applies it only to man. There is not in the paper the least hint that the author ever thought of generalising the remarkable sentence quoted above.
Of Mr Patrick Matthew, who buried his treasure in an appendix to a work on "Naval Timber and Arboriculture", Darwin said that "he clearly saw the full force of the principle of natural selection." In 1860 Darwin wrote--very characteristically--about this to Lyell: "Mr Patrick Matthew publishes a long extract from his work on "Naval Timber and Arboriculture", published in 1831, in which he briefly but completely antic.i.p.ates the theory of Natural Selection. I have ordered the book, as some pa.s.sages are rather obscure, but it is certainly, I think, a complete but not developed antic.i.p.ation. Erasmus always said that surely this would be shown to be the case some day. Anyhow, one may be excused in not having discovered the fact in a work on Naval Timber." ("Life and Letters" II. page 301.)
De Quatref.a.ges and De Varigny have maintained that the botanist Naudin stated the theory of evolution by natural selection in 1852. He explains very clearly the process of artificial selection, and says that in the garden we are following Nature"s method. "We do not think that Nature has made her species in a different fashion from that in which we proceed ourselves in order to make our variations." But, as Darwin said, "he does not show how selection acts under nature." Similarly it must be noted in regard to several pre-Darwinian pictures of the struggle for existence (such as Herder"s, who wrote in 1790 "All is in struggle... each one for himself" and so on), that a recognition of this is only the first step in Darwinism.
Profs. E. Perrier and H.F. Osborn have called attention to a remarkable antic.i.p.ation of the selection-idea which is to be found in the speculations of Etienne Geoffroy St Hilaire (1825-1828) on the evolution of modern Crocodilians from the ancient Teleosaurs. Changing environment induced changes in the respiratory system and far-reaching consequences followed. The atmosphere, acting upon the pulmonary cells, brings about "modifications which are favourable or destructive ("funestes"); these are inherited, and they influence all the rest of the organisation of the animal because if these modifications lead to injurious effects, the animals which exhibit them perish and are replaced by others of a somewhat different form, a form changed so as to be adapted to (a la convenance) the new environment."
Prof. E.B. Poulton ("Science Progress", New Series, Vol. I. 1897. "A Remarkable Antic.i.p.ation of Modern Views on Evolution". See also Chap. VI. in "Essays on Evolution", Oxford, 1908.) has shown that the anthropologist James Cowles Prichard (1786-1848) must be included, even in spite of himself, among the precursors of Darwin. In some pa.s.sages of the second edition of his "Researches into the Physical History of Mankind" (1826), he certainly talks evolution and antic.i.p.ates Prof.
Weismann in denying the transmission of acquired characters. He is, however, sadly self-contradictory and his evolutionism weakens in subsequent editions--the only ones that Darwin saw. Prof. Poulton finds in Prichard"s work a recognition of the operation of Natural Selection.
"After enquiring how it is that "these varieties are developed and preserved in connection with particular climates and differences of local situation," he gives the following very significant answer: "One cause which tends to maintain this relation is obvious. Individuals and families, and even whole colonies, perish and disappear in climates for which they are, by peculiarity of const.i.tution, not adapted. Of this fact proofs have been already mentioned."" Mr Francis Darwin and Prof.
A.C. Seward discuss Prichard"s "antic.i.p.ations" in "More Letters of Charles Darwin", Vol. I. page 43, and come to the conclusion that the evolutionary pa.s.sages are entirely neutralised by others of an opposite trend. There is the same difficulty with Buffon.
Hints of the idea of Natural Selection have been detected elsewhere.
James Watt (See Prof. Patrick Geddes"s article "Variation and Selection", "Encyclopaedia Britannica (9th edition) 1888.), for instance, has been reported as one of the antic.i.p.ators (1851). But we need not prolong the inquiry further, since Darwin did not know of any antic.i.p.ations until after he had published the immortal work of 1859, and since none of those who got hold of the idea made any use of it.
What Darwin did was to follow the clue which Malthus gave him, to realise, first by genius and afterwards by patience, how the complex and subtle struggle for existence works out a natural selection of those organisms which vary in the direction of fitter adaptation to the conditions of their life. So much success attended his application of the Selection-formula that for a time he regarded Natural Selection as almost the sole factor in evolution, variations being pre-supposed; gradually, however, he came to recognise that there was some validity in the factors which had been emphasized by Lamarck and by Buffon, and in his well-known summing up in the sixth edition of the "Origin" he says of the transformation of species: "This has been effected chiefly through the natural selection of numerous successive, slight, favourable variations; aided in an important manner by the inherited effects of the use and disuse of parts; and in an unimportant manner, that is, in relation to adaptive structures, whether past or present, by the direct action of external conditions, and by variations which seem to us in our ignorance to arise spontaneously."
To sum up: the idea of organic evolution, older than Aristotle, slowly developed from the stage of suggestion to the stage of verification, and the first convincing verification was Darwin"s; from being an a priori antic.i.p.ation it has become an interpretation of nature, and Darwin is still the chief interpreter; from being a modal interpretation it has advanced to the rank of a causal theory, the most convincing part of which men will never cease to call Darwinism.
III. THE SELECTION THEORY, By August Weismann.
Professor of Zoology in the University of Freiburg (Baden).
I. THE IDEA OF SELECTION.
Many and diverse were the discoveries made by Charles Darwin in the course of a long and strenuous life, but none of them has had so far-reaching an influence on the science and thought of his time as the theory of selection. I do not believe that the theory of evolution would have made its way so easily and so quickly after Darwin took up the cudgels in favour of it, if he had not been able to support it by a principle which was capable of solving, in a simple manner, the greatest riddle that living nature presents to us,--I mean the purposiveness of every living form relative to the conditions of its life and its marvellously exact adaptation to these.
Everyone knows that Darwin was not alone in discovering the principle of selection, and that the same idea occurred simultaneously and independently to Alfred Russel Wallace. At the memorable meeting of the Linnean Society on 1st July, 1858, two papers were read (communicated by Lyell and Hooker) both setting forth the same idea of selection. One was written by Charles Darwin in Kent, the other by Alfred Wallace in Ternate, in the Malay Archipelago. It was a splendid proof of the magnanimity of these two investigators, that they thus, in all friendliness and without envy, united in laying their ideas before a scientific tribunal: their names will always shine side by side as two of the brightest stars in the scientific sky.
But it is with Charles Darwin that I am here chiefly concerned, since this paper is intended to aid in the commemoration of the hundredth anniversary of his birth.
The idea of selection set forth by the two naturalists was at the time absolutely new, but it was also so simple that Huxley could say of it later, "How extremely stupid not to have thought of that." As Darwin was led to the general doctrine of descent, not through the labours of his predecessors in the early years of the century, but by his own observations, so it was in regard to the principle of selection. He was struck by the innumerable cases of adaptation, as, for instance, that of the woodp.e.c.k.e.rs and tree-frogs to climbing, or the hooks and feather-like appendages of seeds, which aid in the distribution of plants, and he said to himself that an explanation of adaptations was the first thing to be sought for in attempting to formulate a theory of evolution.
But since adaptations point to CHANGES which have been undergone by the ancestral forms of existing species, it is necessary, first of all, to inquire how far species in general are VARIABLE. Thus Darwin"s attention was directed in the first place to the phenomenon of variability, and the use man has made of this, from very early times, in the breeding of his domesticated animals and cultivated plants. He inquired carefully how breeders set to work, when they wished to modify the structure and appearance of a species to their own ends, and it was soon clear to him that SELECTION FOR BREEDING PURPOSES played the chief part.
But how was it possible that such processes should occur in free nature? Who is here the breeder, making the selection, choosing out one individual to bring forth offspring and rejecting others? That was the problem that for a long time remained a riddle to him.
Darwin himself relates how illumination suddenly came to him. He had been reading, for his own pleasure, Malthus" book on Population, and, as he had long known from numerous observations, that every species gives rise to many more descendants than ever attain to maturity, and that, therefore, the greater number of the descendants of a species perish without reproducing, the idea came to him that the decision as to which member of a species was to perish, and which was to attain to maturity and reproduction might not be a matter of chance, but might be determined by the const.i.tution of the individuals themselves, according as they were more or less fitted for survival. With this idea the foundation of the theory of selection was laid.
In ARTIFICIAL SELECTION the breeder chooses out for pairing only such individuals as possess the character desired by him in a somewhat higher degree than the rest of the race. Some of the descendants inherit this character, often in a still higher degree, and if this method be pursued throughout several generations, the race is transformed in respect of that particular character.
NATURAL SELECTION depends on the same three factors as ARTIFICIAL SELECTION: on VARIABILITY, INHERITANCE, and SELECTION FOR BREEDING, but this last is here carried out not by a breeder but by what Darwin called the "struggle for existence." This last factor is one of the special features of the Darwinian conception of nature. That there are carnivorous animals which take heavy toll in every generation of the progeny of the animals on which they prey, and that there are herbivores which decimate the plants in every generation had long been known, but it is only since Darwin"s time that sufficient attention has been paid to the facts that, in addition to this regular destruction, there exists between the members of a species a keen compet.i.tion for s.p.a.ce and food, which limits multiplication, and that numerous individuals of each species perish because of unfavourable climatic conditions. The "struggle for existence," which Darwin regarded as taking the place of the human breeder in free nature, is not a direct struggle between carnivores and their prey, but is the a.s.sumed compet.i.tion for survival between individuals OF THE SAME species, of which, on an average, only those survive to reproduce which have the greatest power of resistance, while the others, less favourably const.i.tuted, perish early. This struggle is so keen, that, within a limited area, where the conditions of life have long remained unchanged, of every species, whatever be the degree of fertility, only two, ON AN AVERAGE, of the descendants of each pair survive; the others succ.u.mb either to enemies, or to disadvantages of climate, or to accident. A high degree of fertility is thus not an indication of the special success of a species, but of the numerous dangers that have attended its evolution. Of the six young brought forth by a pair of elephants in the course of their lives only two survive in a given area; similarly, of the millions of eggs which two thread-worms leave behind them only two survive. It is thus possible to estimate the dangers which threaten a species by its ratio of elimination, or, since this cannot be done directly, by its fertility.
Although a great number of the descendants of each generation fall victims to accident, among those that remain it is still the greater or lesser fitness of the organism that determines the "selection for breeding purposes," and it would be incomprehensible if, in this compet.i.tion, it were not ultimately, that is, on an average, the best equipped which survive, in the sense of living long enough to reproduce.
Thus the principle of natural selection is THE SELECTION OF THE BEST FOR REPRODUCTION, whether the "best" refers to the whole const.i.tution, to one or more parts of the organism, or to one or more stages of development. Every organ, every part, every character of an animal, fertility and intelligence included, must be improved in this manner, and be gradually brought up in the course of generations to its highest attainable state of perfection. And not only may improvement of parts be brought about in this way, but new parts and organs may arise, since, through the slow and minute steps of individual or "fluctuating"
variations, a part may be added here or dropped out there, and thus something new is produced.
The principle of selection solved the riddle as to how what was purposive could conceivably be brought about without the intervention of a directing power, the riddle which animate nature presents to our intelligence at every turn, and in face of which the mind of a Kant could find no way out, for he regarded a solution of it as not to be hoped for. For, even if we were to a.s.sume an evolutionary force that is continually transforming the most primitive and the simplest forms of life into ever higher forms, and the h.o.m.ogeneity of primitive times into the infinite variety of the present, we should still be unable to infer from this alone how each of the numberless forms adapted to particular conditions of life should have appeared PRECISELY AT THE RIGHT MOMENT IN THE HISTORY OF THE EARTH to which their adaptations were appropriate, and precisely at the proper place in which all the conditions of life to which they were adapted occurred: the humming-birds at the same time as the flowers; the trichina at the same time as the pig; the bark-coloured moth at the same time as the oak, and the wasp-like moth at the same time as the wasp which protects it. Without processes of selection we should be obliged to a.s.sume a "pre-established harmony" after the famous Leibnitzian model, by means of which the clock of the evolution of organisms is so regulated as to strike in exact synchronism with that of the history of the earth! All forms of life are strictly adapted to the conditions of their life, and can persist under these conditions alone.
There must therefore be an intrinsic connection between the conditions and the structural adaptations of the organism, and, SINCE THE CONDITIONS OF LIFE CANNOT BE DETERMINED BY THE ANIMAL ITSELF, THE ADAPTATIONS MUST BE CALLED FORTH BY THE CONDITIONS.
The selection theory teaches us how this is conceivable, since it enables us to understand that there is a continual production of what is non-purposive as well as of what is purposive, but the purposive alone survives, while the non-purposive perishes in the very act of arising.
This is the old wisdom taught long ago by Empedocles.
II. THE LAMARCKIAN PRINCIPLE.
Lamarck, as is well known, formulated a definite theory of evolution at the beginning of the nineteenth century, exactly fifty years before the Darwin-Wallace principle of selection was given to the world.
This brilliant investigator also endeavoured to support his theory by demonstrating forces which might have brought about the transformations of the organic world in the course of the ages. In addition to other factors, he laid special emphasis on the increased or diminished use of the parts of the body, a.s.suming that the strengthening or weakening which takes place from this cause during the individual life, could be handed on to the offspring, and thus intensified and raised to the rank of a specific character. Darwin also regarded this LAMARCKIAN PRINCIPLE, as it is now generally called, as a factor in evolution, but he was not fully convinced of the transmissibility of acquired characters.
As I have here to deal only with the theory of selection, I need not discuss the Lamarckian hypothesis, but I must express my opinion that there is room for much doubt as to the cooperation of this principle in evolution. Not only is it difficult to imagine how the transmission of functional modifications could take place, but, up to the present time, notwithstanding the endeavours of many excellent investigators, not a single actual proof of such inheritance has been brought forward.
Semon"s experiments on plants are, according to the botanist Pfeffer, not to be relied on, and even the recent, beautiful experiments made by Dr Kammerer on salamanders, cannot, as I hope to show elsewhere, be regarded as proof, if only because they do not deal at all with functional modifications, that is, with modifications brought about by use, and it is to these ALONE that the Lamarckian principle refers.
III. OBJECTIONS TO THE THEORY OF SELECTION.
(a) Saltatory evolution.
The Darwinian doctrine of evolution depends essentially on THE c.u.mULATIVE AUGMENTATION of minute variations in the direction of utility. But can such minute variations, which are undoubtedly continually appearing among the individuals of the same species, possess any selection-value; can they determine which individuals are to survive, and which are to succ.u.mb; can they be increased by natural selection till they attain to the highest development of a purposive variation?
To many this seems so improbable that they have urged a theory of evolution by leaps from species to species. Kolliker, in 1872, compared the evolution of species with the processes which we can observe in the individual life in cases of alternation of generations. But a polyp only gives rise to a medusa because it has itself arisen from one, and there can be no question of a medusa ever having arisen suddenly and de novo from a polyp-bud, if only because both forms are adapted in their structure as a whole, and in every detail to the conditions of their life. A sudden origin, in a natural way, of numerous adaptations is inconceivable. Even the degeneration of a medusoid from a free-swimming animal to a mere brood-sac (gonoph.o.r.e) is not sudden and saltatory, but occurs by imperceptible modifications throughout hundreds of years, as we can learn from the numerous stages of the process of degeneration persisting at the same time in different species.