1 8 PROBLEMS OF GENETICS 



The control of Selection is loose while the conformity to 

 specific distinction is often very strict and precise, and no less 

 so even when several closely related species co-exist in the same 

 area and in the same circumstances. 



The theory of Selection fails at exactly the point where it 

 was devised to help: Specific distinction. 



Let us examine a somewhat different set of facts in the case 

 of another pair of nearly allied species Lychnis diurna and ves- 

 pertina. The two plants have much in common. Both are 

 dioecious perennials, with somewhat similar flowers, the one 

 crimson, the other white. Each however has its peculiarities 

 which are discernible in almost any part of its structure, whether 

 flower, leaf, fruit or seed, distinctions which would enable a 

 person thoroughly familiar with the plants to determine at once 

 from which species even a small piece had been taken. There 

 is so much resemblance however as readily to support the surmise 

 that the two were mere varieties of one species. Bentham, 

 following Linnaeus, in fact actually makes this suggestion, 

 with what propriety we will afterwards consider. Now this case 

 is typical of many. The two forms have a wide distribution, 

 occurring sometimes separately, sometimes in juxtaposition. 

 L. diurna is a plant of hedgerows and sheltered situations. L. 

 ves pertina is common in fields and open spaces, where diurna 

 is hardly ever found; but not rarely vespertina occurs in associa- 

 tion with diurna in the places which that plant frequents. In 

 this case I do not doubt that we have to do with organisms of 

 somewhat different aptitudes. That L. vespertina has powers 

 which diurna has not is shown very clearly by the fact that 

 diurna is sometimes entirely absent from areas where vespertina 

 can abound. 9 But in order to understand the true genetic 

 relations of the two plants to each other it is necessary to observe 

 their behaviour when they meet as they not unfrequently do. 



9 In Cambridgeshire for example vespertina is common but diurna is absent. 

 Whether this absence is connected with the general presence of chalk I cannot say. 

 When introduced artificially diurna establishes itself, for a time at least, without any 

 apparent difficulty and occasionally escapes from the garden on to the neighbouring 

 roadside. 



