326 THE EFFECTS OF CONDITIONS 



the percentage variations, on these values, of the cor- 

 responding values for the Northern races (from North- 

 ern States, Massachusetts, and Southern New Eng- 

 land). On an average fifteen specimens were measured 

 in each case, the extreme numbers varying between 6 

 and 40. As regards body length, we see that the North- 

 ern forms invariably exceeded the Southern, the aver- 

 age difference amounting to 5.1 per cent. In alar extent 

 they were likewise invariably greater, the average ex- 

 cess being 7.0 per cent. In tail measurement, how- 

 ever, the difference was not nearly so constant, it being 

 greater in the Southern races than in the Northern in 

 four out of the ten sets of measurements, whilst the 

 average excess amounted to only 2.9 per cent. 



Accompanying the increase in size of the Northern 

 forms, Allen finds that, as a rule, there is an apprecia- 

 ble decrease in colour. In the South, dark-coloured 

 birds, such as the red-winged blackbird, become 

 blacker. The slaty and olive tints of other birds, and 

 the various shades of red and yellow, become far more 

 intense as one proceeds south, and the pigmentation of 

 the bill and feet also increases. Allen says " the dif- 

 ference in colour between the extremely Northern 

 and extremely Southern representatives of a given 

 species is often so great that, taken in connection 

 with other differences, as in the general size and the 

 size and form of the bill, the two extremes might 

 excusably be taken for distinct species." The size of 

 the bill varies, as a rule, in the inverse ratio to the 

 size of the body, and " in many species there is not 

 only a marked relative increase in the size of the bill 

 to the southward, but in some an absolute increase, 



