351 



AGE AND AREA. 



By way of introduction, I offer brief extracts from " Is the Theory of Natural 

 Selection Adequate ? : ' by Dr. John C. Willis, F.R.S. (" Nineteenth Century," October, 

 1922). 



The general principle of Age and Area has, as a necessary consequence, that the 

 minimum area goes with the minimum age.' But as this minimum area may easily be 

 only a few square yards, inhabited by a very few individuals, it is evident that a species 

 probably arises as very few, and will have a very strenuous struggle for existence to 

 become established. If in any way ill-suited to the conditions that occurred at its place 

 of birth, it will be at once killed out by natural selection. 



The bearings of Age and Area may be easily understood by taking a simple case 

 in geographical distribution. Dr. Willis takes the Ceylon genus Coitus (he was for many 

 years in Ceylon). The four Ceylon species (A, B, C, D) are found — 



In Ceylon. Outside Ceylon. 



A. On the summit of Ritigala. Nowhere. 



B. In all the mountains. Nowhere. 



C. Mountains and wet plains. South India. 



D. Mountains, wet and dry plains. Tropical Asia and Africa. 



The only reasonable explanation of these striking and widespread facts, 

 according to Dr. Willis, seemed to be that distribution depended upon the time that 

 had elapsed since the arrival of the plant in the country under consideration. The 

 species that occurred beyond the Indian peninsula being on the whole the oldest (each 

 in its own group of relationship), would be the first to arrive in Ceylon, and so would 

 have time to spread there in the maximum degree. Somewhere in the south they would 

 give rise to the species now confined to Ceylon and South India, which, being younger, 

 would spread to a less degree in Ceylon ; and finally they would give rise to the species 

 now local in Ceylon, which, being the youngest of all, would have spread least in the 

 island (each, of course, being considered only with respect to its immediate relatives). 

 The Ceylon local species are thus to be looked upon, not as failures, or as special local 

 adaptations to local conditions, but as species occupying small areas because they 

 have not had time to occupy larger. (See note on Eucalyptus pulverulenta, &c, 

 below, J.H.M.). Age does nothing in itself, but it allows time for the various active 

 factors in distribution to produce their effect. Age and Area, as a principle, goes to 

 show, however, that differences between species, generally expressed in differences 

 in structure, count for little, as far as distribution is concerned. 



Upon this view, to return to the Colei of Ceylon, D is a very old species, and 

 reached Ceylon first of all, while C followed soon after. B is younger again, and has 

 only been able to spread over the Ceylon mountains, while A is youngest of all, and has 

 F 



