170 NA TUBAL SCIENCE [September 



" The greatest clanger," writes Dr Wallace, " to a species under new 

 and adverse conditions is, that it should not be able to adapt itself 

 to thein with sufficient rapidity. It is for this reason that, as 

 Darwin concludes, new species arise from those which have a large 

 population, which occupy a wide area, and which present much varia- 

 tion, a combination . . . rarely found except in continental areas." l 

 How far is this hypothesis borne out by facts ? As a matter of fact 

 the majority of species of a country have not a large population, 

 nevertheless many of such species have varied as much as, if not 

 more, indeed, than the more gregarious species with large popula- 

 tions ; thus, it is easy to think of plants, of which large populations 

 exist, generally gregarious, and therefore supplying the primary 

 condition supposed to be requisite for natural selection ; but the 

 remarkable feature about them is that they have never been known 

 to vary ! Thus, Sir J, D. Hooker gives no varieties whatever to any 

 of the following species, Ranunculus Jicaria, Caltha palustris, 

 Lychnis diurna, Erica cinerea, Bcllis pcrcnnis, Urtica dioica, Galium, 

 verum, Scilla nutans, Lemna minor, Pteris aquilina, &c. And 

 when we cross the channel (though England is really or physically 

 part of the Continent) we find no more signs of variation there, 

 whether in France, Germany, Switzerland, the Tyrol, &c. 



On the other hand, take an extremely common plant, Polygonum 

 avictdarc ; though abundant, it is scarcely a social plant, at least, to 

 the extent of those mentioned. It produces several varieties, but 

 are they found in the midst of the commonest, say, the roadside 

 type ? Sir J. D. Hooker says : — " Var. P. littorale (littoral) a 

 passage to P. maritimum (maritime) ; Var. agrestinum (field 

 form) ; Var. arenastrum (sand-loving form) ; Var. rurivagum (way- 

 side form) ; " hence these varieties are not found in the midst of 

 the commonest form, but away from it, in localities characterised 

 by special physical features. In other words, these varieties arise 

 by self-adaptation to their special environments, respectively. 



The second condition requisite for variations consists of " changed 

 conditions of life." Both Darwin and Dr Wallace admit that " a 

 change of climate and food " is requisite for a new variety to arise 

 among the parent type. The latter writer says : — " Now let some 

 important change occur, either in climate, in abundance of food, or 

 by the irruption of some new and hitherto unknown enemies, a 

 change which at first injuriously affects the species." 2 Similarly 

 Darwin writes : — " Let the external conditions of the country alter," 

 and again, "take the case of a country undergoing some . . . change." 3 



The question at once arises, where and when do we find these 

 changes occurring in, or coming to, any particular district, where 

 some species with a large population happens to be ? Is nature 



1 Lot: cit., p. 484. - Loc. tit., p. 483. 3 "Origin, etc.," p. 63. 



