INDIVIDUALS AND SPECIES 43 



across the continent and found a gradual transition 

 from one extreme to the other both in superficial 

 characters and in genitalia. This is only one ex- 

 ample among many, but it is an excellent illustra- 

 tion. The individuals of California are never di- 

 rectly associated with those of Iowa or New York. 

 In any of the three states they are as independent 

 as if they were actually distinct species, conse- 

 quently they respond to the conditions of their 

 own limited environments and hand down through 

 heredity such peculiarities as their heritages dis- 

 play, maintaining distinctive characters of geo- 

 graphic races. It is possible to demonstrate their 

 relationship by tracing the gradual modification 

 of the species through the intervening regions, 

 where even reproductive association maintains an 

 indirect connection between the extremes. 



MacDougal *^ has produced a similar effect by 

 raising Scrophularia leporella^ which grows nor- 

 mally in the mountains of Arizona, at the Desert 

 Laboratory near Tucson, and near the sea. The 

 plant usually has a "strict, scarcely branching 

 shoot with a few fleshy, succulent roots," but at 

 the Desert Laboratory it branched more profusely 

 and formed more roots, and in the maritime loca- 

 tion it formed such extreme growth that some 

 shoots died early for lack of nourishment, and the 

 roots became a great mass. 



" Am. Nat., Vol. XLV, pp. 5-40, 1911. 



