Climate and Time in Their Geological Relations / A Theory of Secular Changes of the Earth's Climate

by James Croll

Ocean-currents in Relation to the Distribution of Plants and Animals.—In the fifth and last editions of the “Origin of Species,” Mr. Darwin has done me the honour to express his belief that the foregoing view regarding alternate cold and warm periods in north and south during the glacial epoch explains a great many facts in connection with the distribution of plants and animals which have always been regarded as exceedingly puzzling.

There are certain species of plants which occur alike in the temperate regions of the southern and northern hemispheres. At the equator these same temperate forms are found on elevated mountains, but not on the lowlands. How, then, did these temperate forms manage to cross the equator from the northern temperate regions to the southern, and vice versâ? Mr. Darwin’s solution of the problem is (in his own words) as follows:—

“As the cold became more and more intense, we know that arctic forms invaded the temperate regions; and from the facts just given, there can hardly be a doubt that some of the more vigorous, dominant, and widest-spreading temperate forms invaded the equatorial lowlands. The inhabitants of these hot lowlands would at the same time have migrated to the tropical and subtropical regions of the south; for the southern hemisphere was at this period warmer. On the decline of the glacial period, as both hemispheres gradually recovered their former temperatures, the northern temperate forms living on the lowlands under the equator would have been driven to their former homes or have been destroyed, being replaced by the equatorial forms returning from the south. Some, however, of the northern temperate forms would almost certainly have ascended any adjoining high land, where, if sufficiently lofty, they would have long survived like the arctic forms on the mountains of Europe.”

“In the regular course of events the southern hemisphere would in its turn be subjected to a severe glacial period, with the northern hemisphere rendered warmer; and then the southern temperate forms would invade the equatorial lowlands. The northern forms which had before been left on the mountains would now descend and mingle with the southern forms. These latter, when the warmth returned, would return to their former homes, leaving some few species on the mountains, and carrying southward with them some of the northern temperate forms which had descended from their mountain fastnesses. Thus we should have some few species identically the same in the northern and southern temperate zones and on the mountains of the intermediate tropical regions” (p. 339, sixth edition).

Additional light is cast on this subject by the results already stated in regard to the enormous extent to which the temperature of the equator is affected by ocean-currents. Were there no transferrence of heat from equatorial to temperate and polar regions, the temperature of the equator, as has been remarked, would probably be about 55° warmer than at present. In such a case no plant existing on the face of the globe could live at the equator unless on some elevated mountain region. On the other hand, were the quantity of warm water which is being transferred from the equator to be very much increased, the temperature of inter-tropical latitudes might be so lowered as easily to admit of temperate species of plants growing at the equator. A lowering of the temperature at the equator some 20° or 30° is all that would be required; and only a moderate increase in the volume of the currents proceeding from the equator, taken in connection with the effects flowing from the following considerations, might suffice to produce that result. During the glacial epoch, when the one hemisphere was under ice and the other enjoying a warm and equable climate, the median line between the trades may have been shifted to almost the tropical line of the warm hemisphere. Under such a condition of things the warmest part would probably be somewhere about the tropic of the warm hemisphere, and not, as now, at the equator; for since all, or nearly all, the surface-water of the equator would then be impelled over to the warm hemisphere, the tropical regions of that hemisphere would be receiving nearly double their present amount of warm water.

Again, as the equatorial current at this time would be shifted towards the tropic of the warm hemisphere, the surface-water would not, as at present, be flowing in equatorial regions parallel to the equator, but obliquely across it from the cold to the warm hemisphere. This of itself would tend greatly to lower the temperature of the equator.

It follows, therefore, as a necessary consequence, that during the glacial epoch, when the one hemisphere was under snow and ice and the other enjoying a warm and equable climate, the temperature of the equator would be lower than at present. But when the glaciated hemisphere (which we may assume to be the northern) began to grow warmer and the climate of the southern or warm hemisphere to get colder, the median line of the trades and the equatorial currents of the ocean also would begin to move back from the southern tropic towards the equator. This would cause the temperature of the equator to rise and to continue rising until the equatorial currents reached their normal position. When the snow began to accumulate on the southern hemisphere and to disappear on the northern, the median line of the trades and the equatorial currents of the ocean would then begin to move towards the northern tropic as they had formerly towards the southern. The temperature of the equator would then again begin to sink, and continue to do so until the glaciation of the southern hemisphere reached its maximum. This oscillation of the thermal equator to and fro across the geographical equator would continue so long as the alternate glaciation of the two hemispheres continued.

This lowering of the temperature of the equator during the severest part of the glacial epoch will help to explain the former existence of glaciers in inter-tropical regions at no very great elevation above the sea-level, evidence of which appears recently to have been found by Professor Agassiz, Mr. Belt, and others.

The glacial epoch may be considered as contemporaneous in both hemispheres. But the epoch consisted of a succession of cold and warm periods, the cold periods of one hemisphere coinciding with the warm periods of the other, and vice versâ.

Migration across the Equator.—Mr. Belt[103] and others have felt some difficulty in understanding how, according to theory, the plants and animals of temperate regions could manage to migrate from one hemisphere to the other, seeing that in their passage they would have to cross the thermal equator. The oscillation to and fro of the thermal equator across the geographical, removes every difficulty in regard to how the migration takes place. When, for example, a cold period on the northern hemisphere and the corresponding warm one on the southern were at their maximum, the thermal equator would by this time have probably passed beyond the Tropic of Capricorn. The geographical equator would then be enjoying a subtropical, if not a temperate condition of climate, and the plants and animals of the northern hemisphere would manage then to reach the equator. When the cold began to abate on the northern and to increase on the southern hemisphere, the thermal equator would commence its retreat towards the geographical. The plants and animals from the north, in order to escape the increasing heat as the thermal equator approached them, would begin to ascend the mountain heights; and when that equator had passed to its northern limit, and the geographical equator was again enjoying a subtropical condition of climate, the plants and animals would begin to descend and pursue their journey southwards as the cold abated on the southern hemisphere.