54 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY, 



DARWIN'S ALTERNATIVE EXPLANATION OF EVOLUTION. 

 By Rev. Prof. G. Henslow, M.A., F.L.S., V.M.H. 



[Read April 27, 1915; Mr. R. C. Notcutt, F.R.H.S., in the Chair.] 



General Contrast between Darwin's "Theory" and the 

 "True Cause" of Evolution. — In my last lecture (p. 47) I showed 

 how Darwin's theory of Evolution by means of Natural Selection is 

 being discarded by many botanists at least, if not yet by zoologists ; for 

 it is easier perhaps to see the inability of natural selection to account 

 for the origin of species among plants than among animals. To-day I 

 shall show how Darwin's alternative explanation — for it is no theory — 

 has proved to be the right and only way by which Nature originates 

 new varieties and species, i.e. irrespective of crossing and hybridizing. 



We will compare the first and last editions of the " Origin " &c. 



When he first conceived the idea of " Descent with Modification," 

 as Evolution was then described, it passed through his mind, when in the 

 Galapagos Islands in 1835, that such was due to changed conditions 

 of life, i.e. not only the indigenous species of animals and plants, but 

 also variations in the introduced "rats" produced by the new and 

 peculiar climate, food, and soil to which they have been subjected.* 

 But he did not then realize how, nor did he formulate any theoretical 

 explanation ; but from 1838 onwards he devoted himself to working 

 out the theory of natural selection after reading Malthus' " Essay on 

 Population." 



Though he based his theory on natural selection, he never altogether 

 abandoned his original idea ; for he concludes the Introduction to ' ' The 

 Origin of Species by means of Natural Selection " with the words : 

 " I am convinced that Natural Selection has been the most important, 

 but not the exclusive means of modification." Elsewhere he says : 

 " To judge how much, in the case of variation, we should attribute to 

 the direct action of heat, moisture, light, food &c. is most difficult. My 

 impression is that with animals such agencies have produced very 

 little direct effect, though apparently more in the case of plants. . . . 

 Some slight amount of change may, I think, be, therefore, attributed 

 to the direct action of the [changed] conditions of life.' "t 



In preparing his work entitled " The Variation of Animals and 

 Plants under Domestication " he became much more certain as to the 

 effects of the environment ; so that he places the two results on a parallel 

 footing and admits that the latter requires no natural selection at all.f 



* Naturalist's Voyage &c. p. 378 (i860), 

 f Origin &c. 1st ed. p. 10 (1859), 

 t Variation &c. ii. p. 271 (1868). 



