But we can imagine Mr. Darwin promptly snatching his pen to show the stupidity of these northern forms in not climbing Popocatepetl or some other lofty mountain in Central America or Mexico, on their retreat before the still advancing thermal equator. But how this would have helped them to cross the geographical equator, we fail to see. When Mr. Darwin, and the eminent corps of geologists and physicists accepting his solution of this "vexed question," can make a "warm term" south _succeed_ a "cold term" north, we shall have no difficulty in solving the problem ourself. But, unfortunately, the two terms--the cold one north and the warm one south--are simultaneous in occurrence, and the same causes which forced these northern invaders into the tropics, when they followed _after_ the thermal equator, would have driven them ignominously back again _before_ it. The climbing of mountains would only have prolonged their disaster. For after the glaciation north comes the glaciation south, and unless our cold temperate zone were pushed down beyond the geographical equator, none of its living forms could ever have reached the corresponding zone in the southern hemisphere. But as this "migration theory" is one of paramount importance to modern science, and especially to "Darwinism," [19] distinctively so called, let us, at the risk of repetition and tediousness, propose a scientific expedition for the better solution of this problem. To do this, we propose to cut loose from our stupid predecessors, the plants and animals, and invite Mr. Darwin and some of his more distinguished European contemporaries, not omitting Professors Gray, Winchell, Yeomans, and some few other American admirers of his, to accompany us on a fresh expedition from the warm and cold temperate zones north to the corresponding zones south, _purely in the interest of science_. To make it certain that the time fixed upon for this "expedition" to start, will not escape their attention, we will state what many of them already well know, that the present eccentricity of the earth's orbit is very low, being only 0.0168, and that, in the year of our Lord 851,800, it will reach its next superior limit, with a few intervening oscillations of such minimum value as to render it hardly worth our while to start before that time.