Mortality among Plants and its Bearing on Natural Selection

Abstract

I HAVE read with great interest the letters from Prof. Salisbury and Dr. J. Phillips which have appeared in NATURE dealing with mortality in plants at different stages of their life histories. The observations are of the greatest importance, but I doubt whether the interpretation advanced by Prof. Salisbury in his letter of May 31, 1930, is supported by the facts. It seems to me that a distinction should be made between mortality and natural selection. The results obtained by Prof. Salisbury with Silene conica L. and Verbascum Thapsus L. would probably have been very similar if all the seeds in each case had been exactly alike, and, in any case, evolution under natural conditions is proceeding so slowly that individuals of any one species or variety in a taxonomic sense are very nearly identical. On the other hand, the seed yield of many individual plants runs into thousands and millions, which, combined with the losses between pollen production and germination, makes the chances against survival into the second generation of any single pair of gametes enormous. How far natural selection enters into this enormous mortality can never be known: for while accidental mortality deals equally with the less and the more favourable variation, competition verging on mortality puts a high premium on favourable variations, however small, as, for example, in competition for light or plant food.