DAP^YIN AND BOTANY 357 



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 Pres- 

 ervation of Favored Eaces 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 con- 

 clusion 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 



