586 



THE AMERICAN NATURALIST 



[Vol. LI 



conclusion seems clear that many marked morphological 

 characters in plants have arisen independently of func- 

 tion and without the aid of natural selection. This con- 

 clusion is all the more probable because form is so much 

 more loosely tied to function in plants than in animals. 

 In many plants it makes little or no difference what is the 

 shape of the leaf so far as its chlorophyllian function is 

 concerned, nor what is the shape of the anthers so long as 

 they produce pollen. 



Another matter, which I have touched upon elsewhere,'* 

 is the geographic relationships of the most closely related 

 species of plants. It appears that Jordan's well-known 

 law that the most nearly related species occupy adjacent 

 areas, although widely applicable especially to the sub- 

 species of mammals and birds, is by no means so generally 

 true in regard to plants. But we shall come to this point 

 again. 



Eeferring now to animals, the North American screech 

 owls afford an interesting case in which two kinds of 

 variability can be clearly contrasted as regards their geo- 

 graphic relationships. These two types of variations 

 are (1) those in which apparently continuous or nearly 

 continuous variations occur progressively over certain 

 geographical areas, with no two forms occupying the 

 same area, and (2) those in which two or more sharply 

 marked forms occupy the same area. 



The accompanying map, compiled largely from Eidg- 

 way's data,** shows the distribution of the various sub- 



5 Gates, E. E., 1916, "On Pairs of Species," Bot. Gazette, 61: 177-212. 

 Figs. 12. 



e-Ridgway, Eobert, 1914, "The Birds of North and Middle America," 

 Bull. U. S. Nat. Mus., No. 50, Part VI, pp. 882, pis. 36. 



Eidgway says (p. 683): "In the main, geographic variations [in Otus] 



relatively short distances, indicating an organization sensitive to slight 



