The thermal equator at length reaches the base of Mt. Tolima, and hastens northward to the Isthmus, and thence to Hondurus and New Guatemala, where, by sheer force of exhaustion, it comes to a halt. But, as the equatorial zone extends fifteen degrees both ways from the thermal equator, its southern limit now rests on the geographical equator, and accordingly encircles the base of our "mount of refuge." We are now up this mountain some sixteen thousand feet above the equatorial lowlands, with the sub-tropical, tropical, and equatorial zones between us and the possibility of our further migration southward, without violating the express conditions imposed at the outset of our expedition. The fact soon stares us in the face that we have been no more successful, in our efforts to cross the thermal equator and pass into high southern latitudes, than the stupid plants and animals before us; and Mr Darwin's faith in high mountains springing from equatorial lowlands, disappears in jest and derision as we all good-humoredly agree "to break conditions," and find our way back to the centres of activity and trade in the Old and New Worlds, leaving the great scientific problem of the ages to solve itself as best it may. We accordingly descend from our mountain fastness, hasten to the coast, and take passage by steamer to Manhattan, the great commercial metropolis of the world. Here we find that the barometer of exchange was long ago taken down in London and hung up in New York. The Old Antiquarian Society rooms are the first object of interest sought by us. On making our way thither we look for a copy of the _Herald_, of the date of our departure, in which we find an account of the scientific expedition fitted out by us, facetiously termed "_The Great Wild-Goose Chase after the Thermal Equator_"--presenting one of the most humorous bits of sensational pleasantry ever given to the American public. But an apology is due the staider reader for the seeming levity of this narrative adventure. The exposition of Mr. Darwin, though widely accepted on both sides of the Atlantic by the scientific world, has seemed to us too trivial for serious reply. If we have leaped over vast periods of time, it makes no difference with the argument. So long as the thermal equator, or more properly the equatorial zone, or any part of it, lies between the warm or cold temperate forms, whether plants or animals, and their point of destination in the southern hemisphere, they can never migrate thither, any more than the right whale of the arctic seas can swim the equatorial oceans. Nothing is gained by going out of the way to climb mountains, except to hopelessly retard the return of both plants and animals to their native zones. If we have not demonstrated this fact to the reader's fullest comprehension, it will be useless for him ever to write a Q.E.D. at the end of any proposition.