436 SECTIONAL TRANSACTIONS.— D. 



such regularities, to be employed as a subsidiary (not alternative) method 

 in taxonomy to that of naming areal groups. This latter method, applied 

 alone, attaches undue importance to named as against unnamed groups, 

 and suggests an unreal uniformity within named groups. The best known 

 cases of clines apply to warm-blooded animals and have been subsumed 

 under the so-called rules of Gloger, Allen, etc. These rules apply only to 

 the means of subspecies or allied species ; but regularities within named areal 

 groups may also occur. Thus clines may be inter- or intra-group. Clines 

 for different characters may run in different directions (shrikes, American 

 sparrows) or in different ways in the same direction (bees). In many cases 

 clines concern physiologically important properties such as temperature- 

 resistance (Drosophila), ecological preference (Plantago), reproductive 

 adaptations (Lymantrta) , etc. When intra-group clines occur, these may 

 be gradual, but joined by steep ' genoclines ' where two subspecies meet. 



It is probable that research will show many more examples of clines, even 

 in apparently uniform populations, though small isolated populations tend 

 to develop ' accidental ' (non-adaptive) characters unrelated to any character- 

 gradient (see Sewall Wright for the theoretical basis of this well-known fact). 



If so, we may envisage small-scale evolution as operating in some such 

 way as this : 



(i) Selection adapts the characters of large populations locally to the 

 environmental conditions, thus producing clines. The slope of the 

 cline will depend on the intensity of selection, the degree of variance 

 available, and the freedom of gene-spread due to interbreeding. 

 The clines may be geographical over wide areas, or ecological over 

 small distances. 



(2) Barriers due to isolation, geographical or ecological (or in some cases 

 genetic), may interrupt the uniform clines. If the barriers are com- 

 plete, the slope of the new intra-group clines will be much reduced 

 due to interruption of gene-flow, though the clines will remain in 

 inter-group form. If the barriers are incomplete (contiguous sub- 

 species) the slope of the intra-group clines will be less reduced, and 

 they will be connected by steep genoclines. 



In addition, the clines will be partially obscured by the fixation of 

 ' accidental ' characters. 



(3) Subsequent migration may still further obscure the original regu- 

 larities. 



Extremely interesting comparative studies could be made of the slope of 

 clines (inter- and intra-group) for the same character in related species. 



Prof. R. A. Fisher, F.R.S. — Selective intensities in nature (10.25). 



Polymorphic species provide an exceptional opportunity for estimating 

 the intensity of selective agencies in nature. Some results are presented, 

 based on the collections of grouse locusts organised by Dr. R. K. Nabours 

 and his associates. 



Prof. A. E. Trueman. — Orthogenesis (11.50). 



The term orthogenesis, proposed by Eimer in 1895, has been variously 

 used by later workers. To some it has implied no more than evolution 

 by definite and successive variations in a given direction, as contrasted with 

 indefinite variation ; but many workers have used the term with some 

 implication of causation, the direction of evolution being supposed to be 



