Now, it is well known that the earth and the solar system in their onward progress pass through trains of meteorites. The tails of some of the comets are indeed pretty clearly proved to be streams of ponderable matter, through which, from time to time, the minor members of the solar system plunge, and receive some accession to their bulk and weight. The shooting-stars, which occasionally attract our attention in the sky, mark the course of such meteorites as they pass through the earth's atmosphere, and are heated to a glow by the friction with it. It has been suggested, therefore, that the sun itself may at times have its amount of heat sensibly affected by such showers of meteorites or asteroids. Upon this theory the warm period of the Tertiary epoch, for instance, may have been due to the heat temporarily added to the sun by impact with minor astronomical bodies. When, afterwards, it gradually cooled down, receiving through a long period no more accessions of heat from that source, the way was prepared for the colder epoch of the Glacial period, which, in turn, was dispelled by fresh showers of meteorites upon the sun, sufficient to produce the amelioration of climate which we experience at the present time. As intimated, this theory is closely allied to the preceding, the principal difference being that it limits the effects of the supposed cause to the solar system, and looks to our sun as the varying source of heat-supply. It has the advantage over that, however, of possessing a more tangible _vera causa_. Meteorites, asteroids, and comets are known to be within this system, and have occasional collisions with other members of it. But the principal objection urged against the preceding theory applies here, also, with equal force. The accumulations of ice during the Glacial period were not determined by latitude. In North America the centre of accumulation was south of the Arctic Circle--a fact which points clearly enough to some other cause than that of a general lowering of the temperature exterior to the earth.