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. To have accomplished this grand glaciatorial feat the North Pole must have donned some twenty times the amount of ice now about both poles of the earth, and so changed the earth's centre of gravity as to have inundated every foot of land on its habitable surface. But if this terrible catastrophy had been avoided, and some of our extreme northern forms had forced their way through the Isthmus into the lowlands of Columbia, they must have done so at their greatest possible peril, even if they had reached the base of Old Mt. Tolima in advance of the thermal equator, now fleeing in dismay before the southern Ice-monarch, with all his isochimenal hosts in mad pursuit of their invaders. And if these adventurous northern forms had succeeded in ascending Mt. Tolima, they could never have got down again, with the assistance of forty glaciations.