CORRESPONDENCE. 



Sir — Mr. Bulman's paper in your February number reopens a very interest- 

 ing question. In the paper which he criticises, I contended (1) that nearly 

 allied species are intersterile, and (2) that this being granted, we may put 

 down to the credit of insects the development of flowers. To make good my 

 first contention I depended mainly on the bees themselves ; they not infre- 

 quently wander from species to species, and yet no intercrossing takes place. 

 Thus I am able to dispense with the evidence supplied by the experiments of 

 Alexis Jordan. If, however, his conclusions are accepted, my case becomes 

 still stronger, since intersterility is extended even to sub-varieties. If it be 

 conceded that species are sterile inter se, it is not difficult to show the reason- 

 ableness of my second contention. The plants compete for the visits of bees, 

 and the bees are in the position of a gardener who isolates a particular species, 

 selects his plants of brightest bloom, and sows seed only from them. This 

 assumes that bees have a colour -sense. The experiments by which Prof. 

 Plateau attempted to prove that they had none really proves nothing of the 

 kind, as Sir John Lubbock made clear in the Journal of the Linnaean Society 

 (April 1, 1898). It is true that scent may answer the same purpose as colour, 

 but colour or fragrance a plant must have or else bees will leave it unvisited. 

 As to the term by-product, I meant that the plants from which our phanerogams 

 are descended produced colour, but turned it to no useful purpose. It has 

 been fostered and developed through natural selection. 



There remains a point which Mr. Bulman has raised and which I certainly 

 dealt with inadequately. "Admitting," he says, "that all species, sub-species, 

 and varieties, as they exist to-day, are sterile inter se, we cannot suppose that 

 the varying individuals in a species — -which must form the beginning of a new 

 species — are so." In this almost all biologists will agree with him. It is 

 impossible to accept the theory of physiological selection in the form in which 

 Romanes propounded it. If a few individuals in a species, having no 

 superiority to their fellows and no distinguishing mark, are fertile only inter se, 

 they are not likely to leave descendants : with flowering plants, since in many 

 species cross-fertilisation is required only occasionally, the chances are much 

 better. The few " physiologically separate " individuals might, by means of 

 self-fertilised ovules, so increase their numbers as no longer to be scattered 

 units among the herd from which they have cut themselves off. Setting aside 

 this possibility, we can appeal to geographical isolation to help us. Since 

 individual plants are fixed in one spot, such isolation may easily arise and 

 continue long enough for a variety, originated locally, to become sterile with 

 other varieties or with the parent species. Such local forms might arise in 

 neighbouring valleys or on the banks of two streams that flowed not far apart. 

 Armed with the intersterility thus obtained our young species will extend their 

 range and settle among allied species and varieties without danger of inter- 

 crossing, and the bees will be able to pursue the work of developing their 

 flowers. F. W. Headley. 



Haileybury College, Hertford. 



250 



