trees is proved by their size, some being four feet in diameter. Round them, as they gradually went down with the subsiding soil, calamites grew at one level after another. In the Sidney coal-field fifty-nine fossil forests thus occur in superposition. [Sidenote: Organic proofs of a former high temperature.] Such was the conclusion forcing itself from considerations connected with inorganic nature. It received a most emphatic endorsement from the organic world, for there is an intimate connexion between the existence and well-being both of plants and animals, and the heat to which they are exposed. Why is it that the orange and lemon do not grow in New York? What is it that would inevitably ensue if these exotics were exposed to a cold winter? What must take place if, in Florida or other of the Southern states, a season of unusual rigor should occur? Does not heat thus confine within a fixed boundary the spread of these plants? And so, again, how many others there are which grow luxuriantly in a temperate climate, but are parched up and killed if fortuitously carried beneath a hot tropical sun. To every one there is a climate which best suits the condition of its life, and certain limits of heat and cold beyond which its existence is not possible. If the mean annual heat of the earth's surface were slowly to rise, and, in the course of some centuries, the temperature now obtaining in Florida should obtain in New York, the orange and lemon would certainly be found here. [Sidenote: Boundary of organisms by heat.] With the increasing heat those plants would commence a northward march, steadily advancing as opportunity was given. Or, if the reverse took place, and for any reason the heat of the torrid zone declined until the winter's cold of New York should be at last reached under the equator, as the descent went on the orange and lemon would retreat within a narrow and narrower region, and end by becoming extinct, the conditions of their exposure being incompatible with the continuance of their life. From such considerations it is therefore obvious that not only does heat arrange the limits of the distribution of plants, erecting round them boundaries which, though invisible, are more insuperable than a wall of brass, it also regulates their march, if march there is to be--nay, even controls their very existence, and to genera, and species, and individuals appoints a period of duration.