Russel Wallace : Alfred Russell Wallace (sic)
Precession of the Equinoxes, Illustrating Their
Relation to Geological Climate and the
Rate of Organic Change (S171: 1870)
At the recent meeting of the British Association, Mr. A. R. Wallace read a paper on the "Earth's Eccentricity and the Precession of the Equinoxes," illustrating their relation to geological climates and the rate of organic change. The paper was illustrated by a diagram constructed by means of the tables published by Mr. Croll in the Philosophical Magazine of August, 1868, and covered a period of 3,000,000 of years before A.D. 1800, and 1,000,000 years after that date. Mr. Wallace stated that with the existing amount of eccentricity the earth was nearer the sun in winter than in summer in about the proportion of 90 to 93, and received, therefore, about one-seventeenth more heat in December than in June, but the difference was neutralized by the peculiar distribution of land and sea. When the eccentricity of the earth was great it would cause an enormous difference in the climates, causing the winter to be lengthened and the summer to be correspondingly shortened; but, on the other hand, during the periods of low eccentricity the climate would change but little. The conclusion drawn in regard to the glacier epochs was that it was evident that, if these causes had had any important share in producing them, then the indications that geologists had found of intercalated warm periods represented not an unusual or abnormal event, to be speculated on as something extraordinary and difficult to account for, but the normal state of things; and that a consequent migration of species, of which evidence was also found, must have occurred, not once only, but over and over again during the whole continuance of the glacier period. A full appreciation of this would, perhaps, assist in the interpretation of many well-known anomalies of distribution. A still more important conclusion was to be drawn from the almost endless succession of migrations of animals and plants that must have been caused by these often-repeated changes of climate--migrations that would take place whenever the change of climate was considerable, even though no glacier epoch appeared. These migrations being interrupted in various directions by seas, mountains, or other obstacles, would inevitably lead to the crowding together of animals and plants before separated, and thus lead to a complex and often renewed struggle for existence, and give wide scope for new and favourable variations to establish themselves. One of Mr. Darwin's great principles was that "the relation of organism to organism was the most important of all relations," and, "that though a change of climate may produce a considerable direct effect in modifying species, it induces far greater indirect effect by favouring the increase or diminution of enemies and competitors of the species." He also assured Mr. Wallace that there was no cause so powerful in inducing specific changes as mutations of climate through the consequent migrations. He also found from the diagram that we live, and have done for the last 60,000 years, in a period of small eccentricity. During the past 4,000,000 years there were only nine occasions when the eccentricity was so little or less than it is now, while the average is nearly twice as great; for considerable periods it was three times as great. It happened that for the last 60,000 years there had been but little mutation of climate each 10,000 years--the periods into which he divided the 4,000,000--and, therefore, in Mr. Darwin's own words, "the most powerful cause in inducing specific changes" had been in abeyance. Any estimate we might form of the rate of specific change from the stability of species during that period would therefore be fallacious. The period of about 10,000 years of alternate changes of climate would seem to be one well adapted to favour rapid changes of species, as the change would be sufficiently gradual to allow of any possible amount of migration. There would be sufficient time for the appearance of abundant variation, and for the increase to any extent of species adapted to the changed conditions, whilst there would also be time for the new and complete relations into which they would be thrown to become adjusted and balanced. If we are adequately impressed with the highly complex relations which subsist between each organism and all around it--which Mr. Darwin has done so much to elucidate--and if we further accept his views that all changes in these relations, however brought about, necessarily cause the modification of some forms and the extinction of others, it seems hardly possible to conceive a state of things better adapted to promote the increasing growth and change of the organic world than that which he presents to us.