Page:Popular Science Monthly Volume 74.djvu/361

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357
DARWIN AND BOTANY

earlier stages of its existence. The facts of geographic distribution were eagerly examined as bearing on the theory of descent and Darwin's writings abound in speculations relative to their significance. He was inclined to combat the geologic theory of former land connections of present existing continents, as not satisfactorily accounting for many features of geographic distribution, though he ultimately agreed with this theory to some extent. He closely studied the natural means by which seeds are transported over great distances and also inquired into the vitality of seeds.

The title of the "Origin" was a subject of considerable doubt in his mind, and in 1857, two years before it was printed, he had proposed to call it "Natural Selection." The title "Origin of Species by Means of Natural Selection," is, if taken literally, somewhat misleading, and has occasioned considerable discussion. The subtitle—" Or the Preservation of Favored Races in the Struggle for Life "—is a more accurate statement of his theory. On November 23, 1856, he wrote to Dr. Hooker:

The formation of a strong variety, or species, I look at as almost wholly due to the selection of what may be incorrectly called chance variations. Again, the slight differences selected, by which a race or species is at last formed, stand, as I think can be shown in the far more important relation to its associates than to external conditions.

Darwin's great contribution to the subject of evolution was the incontrovertible proof adduced by him that living species are modified descendants of preexisting species, and that the modifications are brought about by natural causes. His observations led him to the conclusion that the modifications were all minute, gradual and cumulative. We know that they may also be considerable and abrupt and that they are cumulative because favorable changes are preserved.

How, then, do the modifications or primordial variations, either large or small, arise? Is variation an innate essential quality, or is it induced by external environmental factors? Proof of environmental agencies having at least something to do with it in plants seems to be accumulating, as the experimental work carried on by MacDougal and by Gager at the New York Botanical Garden appears to imply.

I think that we may now safely outline the methods of formation of species somewhat as follows: Through causes which are not yet at all well known, but by means of which agencies external to the germ-cells certainly may have a part, the offspring of a plant grown from seed differ more or less from the parent (variation). The thus modified offspring, subjected to natural selection, ultimately perish if they are unadapted, but survive if they are adapted to their surroundings. Repetitions of this process finally bring the descendants of plants to differ materially from their ancestors (evolution). The end of the