EVOLUTION 1051 



variations, on which Darwin laid emphasis. That the natural selec- 

 tion of the variations would not be fortuitous is obvious if the 

 change of climate was drastic enough to make slightly thicker fur 

 of survival value. 



The Darwinian theory, as appHed to natural acclimatisation, may 

 be briefly resumed in more technical terms. In any species in any 

 given environment there will be an observable percentage of 

 variation from type in a given number of offspring produced, 

 and these variations yield a curve of frequency or probable error. 

 The steepness of the curve is a measure of the variability of the 

 species. The less adaptive aberrants on all sides are pruned off by 

 selection. If the environment is changed, selection may no longer 

 operate on the same axis as before, but may tend to prune off 

 variations on one side of the mean more than on another. In the 

 course of time, the apex of the curve, representing the type form, 

 will shift to suit the new conditions, since more of the aberrants 

 on the favoured side will live to reproduce. 



Quite a separate question is this : that the new environment may 

 increase the variability of the species, flattening the curve of pro- 

 bable error. The effect of this is that selection has more material to 

 work with, and therefore attains its end more rapidly, although of the 

 variations appearing as many will be unfavourable as favourable. 

 We may say that a species in which there is little variation presents 

 a greater inertia to the shifting effect of altered climatic conditions 

 than a variable species. In the less variable species there will be 

 relatively fewer of the favourable variations to mate with the 

 strict type form. 



Bateson's Silliman Lectures contain a valuable criticism of the 

 somewhat facilely accepted views (a) that local and climatic varieties 

 are adaptational, {h) that the influences of environment have 

 directly led either to the production of these varieties or to their 

 selective stabilisation, and (c) that there is gradual transition or 

 "mass- transformation" from one species to another in response to 

 climatic influences. Bateson lays much emphasis on the role of 

 isolation and on the intrinsic character, e.g. Mendelian dominance, 

 of the sporadic variations that are of frequent occurrence. 



Kammerer's laborious study (1926) of the species and varieties 

 of lizards on the Dalmatian Islands lays emphasis on the importance 

 of insulation (or isolation) as a segregating factor. Variations in 

 colouring and marking are at first directly induced as physiological 

 reactions, but they become structurally and hereditarily fixed; 

 some have adaptive significance and others none. He gives interest- 

 ing data in regard to the lines or rules of variation which the lizards 

 illustrate. 



Of importance in connection with accHmatisation is Erwin Baur's 

 study of cultivated snapdragons, especially Antirrhinum majus. 



