124 ADAMS. 



in the Appalachian Valley, was an important highway for 

 aquatic life, and the valley remains to the present time a highway 

 for land forms. 



DESIRABILITY OF STUDYING THE VARIATIONS OR MODIFICATIONS IN 



ANIMALS AND PLANTS AS THEY DIVERGE FROM THEIR CENTER 



OF ORIGIN ALONG THEIR LINES OF DISPERSAL. 



With the increased interest which is now being shown in the 

 study of individual variations of plants and animals, it is of value 

 to see the relation which such studies have to the highways of 

 dispersal, and vice versa. The real importance of this relation 

 does not seem to be fully realized. This fact is apparent when 

 the geographical variations are largely expressed in terms of the 

 cardinal points, instead of in their relation or deviation from the 

 center of dispersal or changed environment. On account of the 

 topographical relations and past climatic changes of our country, 

 most of the migrations have been deflected more or less to the 



o 



North or South, and thus such expressions apply in general. 



A few points will be mentioned to which special attention is 

 directed : 



I. Variation in size, its diminution from the central area. 

 Allen ('76, p. 310) has shown that in North America birds and 

 mammals of austral origin tend to increase in size to the South, 

 while those of boreal origin tend to decrease in size. How gen- 

 eral the application of this law is among other animals, has yet to 

 be determined, nor is it known how much the paths of dispersal 

 may influence this. This seems to show concentric divergence 

 from the center of origin. Allen ('71, p. 92) has also called at- 

 tention to important variations in the size of " peripheral organs " 

 in birds, such as the tail, beak and claws. Weed ('92 and '93) 

 has published two interesting papers on the geographical varia- 

 tions of the harvest-spiders, which show that they increase in size 

 of body and length of legs to the South. 



Jordan ('93, p. 23), in speaking of the increase in the number 

 of vertebrae in fish toward the North, says : "In most cases, as 

 the number of vertebrae increases, the body becomes propor- 

 tionally elongate. As a result of this, the fishes of the Arctic 

 waters are, for the most part, long and slender, and not a few of 



