But first let us lay down a few undeniable propositions, on the strength of which this ingenious and purely speculative theory of Mr. Darwin must rest:--
1. It is universally conceded by the scientific world that these glacial epochs, however many of them there may have been in the past and however few there may be in the future, depend, for their occurrence, upon the maxima of eccentricity in the earth's orbit about the sun.
2. The actual amount of heat which the earth annually receives from the sun is in no way affected by the eccentricity of its orbit. It is a constant quantity, and only unequally distributed on the earth's surface, being neither increased nor diminished, as our winters occur in aphelion or perihelion.
3. The actual amount of ice-cap accumulated about the two poles of the earth, is also a constant quantity. And to measure the severity of any glacial epoch, we have only to determine the exact amount of ice (not altogether an impossible problem) about the two poles at any given time, and then determine the effect of its entire transference from one pole to the other.
4. It is not probable that the present ice-cap of the south pole extends continuously and permanently much farther north than 80° or 81°. Mt. Erebus, in Victoria Land, lies in about this latitude, and it was only a few years since that the coast line of that island or continent was traversed, by English exploring vessels, from Mt. Erebus to a point some ten or twelve degrees further north. [[18]]
5. But if we estimate the southern cap as extending continuously to 75°, what would be the effect of its transference at once to the ice-cap of the north pole? Would it extend it, after assuming its proper glacial slope, below 60°, a point falling within the present subarctic zone? The utmost limit to which Mr. Croll, in his great work on "Climate and Time," conceives it possible that it should extend, in any glacial epoch, is to 55°, or about the northern boundary of England.
Now unless the astronomers and physicists are all at sea about the causes of glaciation, the warm temperate zone can never be pushed any further south than the tropical zone, nor the cold temperate any further than the sub-tropical. This would be the extreme limit. Mr. Croll says, in speaking of these glacial periods; "It is, of course, absurd to suppose that an ice-cap could ever actually reach down to the equator. It is probable that the last great ice-cap of the glacial epoch nowhere reached half way to the equator. Our cap (that of Europe) must therefore, terminate at a moderately high latitude." And if the gulf stream flows southward during the glacial period north, as he supposes probable, the cap on this continent would probably terminate at the same moderately high latitude. Assuming that Mr. Croll's estimate is the more probable one, it would only push the cold temperate zone down to the line of the Gulf States; the warm temperate, to the southern line of Mexico; the sub-tropical, to the Central American States, and the tropical to the United States of Columbia, Venezuela, and Guiana.
Suppose, then, that some seven hundred thousand years ago, more or less, when the North Pole had fully donned the earth's ice-cap, with all the isothermal and isochimenal changes thereby effected, what must have been the line of march taken by our northern vegetal and animal forms to escape the cataclysm of ice and snow then impending? Manifestly, they would have flocked, first to the Gulf states, then to Mexico, and afterwards to the Central American states; but none of them could ever have been crowded through the Isthmus of Panama, since at the height of the last glaciation, that portion of the continent must have been the tropical barrier to our northern forms, as it is now the equatorial barrier.
For the sake of the argument, however, we will suppose the northern ice-cap to have been even more imperative in its demands than Mr. Croll has deemed possible, driving some of our warm and cold temperate forms down into the lowlands of Columbia, Venezuela, etc., in the extreme northern portions of South America. But how would these forms have managed, even then, to cross the thermal equator and secure a permanent habitat in the present warm and cold temperate zones of that continent? Manifestly, this question has never been practically solved, nor is it ever likely to be in our day or generation. It is nevertheless susceptible of solution, as Mr. Darwin thinks, by easy mental processes. We have only to take a bird's eye view of the situation, and mentally follow these forms in their long geographical tramp from the northern to the southern hemisphere.
They must have started, of course, some twenty thousand years or more before the earth reached its last superior limit of eccentricity. At that distant epoch the sub-arctic breezes must have been blowing pretty stiffly in our present temperate latitudes, and these forms would have been constrained, in due time, to seek a more congenial isotherm. They must accordingly have set out on their expedition, at about the period indicated, with the prospect of a long and tedious journey before them. Some twenty thousand years must have transpired before they reached the line of the present Gulf states, and it would have taken as many more years for them to deploy to the right and successfully enter the Mexican states. In another twenty thousand years or so they might have doubled Vera Cruz, and headed, in a southeasterly direction, for the Central American states. The thermal equator would by this time have reached a point some thirty degrees south of the geographical equator, while the northern ice-cap would have swept down upon the traditional "hub of the universe," or some ten or twelve degrees in excess of Mr. Croll's calculations.