Page:Popular Science Monthly Volume 9.djvu/704

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THE POPULAR SCIENCE MONTHLY.

We have passed in rapid review the evidence upon which guesses, more or less plausible, as to the age of the world, have been founded. Whatever may be the opinion at which men will ultimately arrive, it cannot but be satisfactory to note from how many quarters and in how many ways Natural Science has in latter days cast light on the inquirer's path.—Quarterly Review.

 

THE LOCAL DISTRIBUTION OF PLANTS AND THE THEORY OF ADAPTATION.
By LESTER F. WARD, A. M.

THERE is one class of facts in the geographical distribution of plants which has not received, at the hands of botanists, the degree of attention which its importance justifies.

I do not refer to those wide general phenomena which a comparison of the floras of different countries renders so striking, and by which the more humble and restricted class to which I would call attention is usually eclipsed. Such general considerations are, it is true, exceedingly interesting and important, and are in no danger of receiving too much attention. Nothing could be more absorbing than a close comparative analysis of the vegetation of different hemispheres, continents, islands, and zones, of the globe. The most casual survey of such fields reveals marvels, the mere acquaintance with which excites in the mind of the botanist the liveliest interest and pleasure. The strange and leafless euphorbias of South Africa, with their naked, green, parenchymous branches; the equally singular and grotesque cactuses of America answering to them; the anomalous vegetation of Australia, with its shadeless forests due to their vertical foliage; the absence of oaks east of the Ural Mountains, and of heaths on this side the Atlantic; the confinement of the genus Rosa to the northern and of the genus Calceolaria to the southern hemisphere—these and numberless other kindred facts connected with the general distribution of plants over the globe are justly calculated to excite the most intense interest, and have given rise to a variety of theories designed to account for them.

The phenomena, however, to which I would more particularly refer, come much nearer home, and may be presumed to have attracted the attention, more or less forcibly, of every one at all conversant with plants. They constitute a distinct class, and may be described in general terms as facts unfavorable to the received theory of adaptation.

It has long been regarded as a law of life, applicable alike to animal and vegetable forms, that each species is exactly adapted to the particular habitat where it occurs; and naturalists, assuming this law,