DISTRIBUTION OF FLORA AND FAUNA. 125 



them approach the form of eels. In the tropics, however, while 

 elongate fishes are common enough, most of them (always ex- 

 cepting the eels) have the normal number of vertebrae, the greater 

 length being due to the elongation of their individual vertebrae, 

 and not to their increase in number." The relation of these 

 changes to paths of dispersal appears to be close, in some cases 

 at least, as Jordan ('01, p. 566) emphasizes that there is a North 

 and South trend of the lines of distribution of shore fishes. 



2. Very closely related to the above law of size and that of 

 dominance of a type in its native home is another formulated by 

 Hyde ('96, p. 575) of the U. S. Department of Agriculture, con- 

 cerning the productiveness of crops. He says : " Entirely inde- 

 pendently of whether the average yield per acre be high or low, 

 the nearer the approach to the region to which a product is in- 

 digenous, the more uniform will be the rate of production from 

 year to year ; and the farther the departure from such region the 

 greater the liability to fluctuation. This law clearly indicates 

 the greater stability in the indigenous area, and increased varia- 

 bility, with departure from that center. The close relation of the 

 law of size and Hyde's law of productiveness indicates a general 

 tendency which is surely of considerable economic as well as 

 scientific importance, especially with regard to the introduction 

 and acclimatization of foreign plants and animals. 



3. The continuity and directness of the individual variations 

 along diverging lines of dispersal is of considerable importance. 

 This fact was impressed upon me by the study of a very exten- 

 sive collection of gasteropod shells from many localities in the 

 Tennessee River system. 1 There are two very distinct types of 

 shells, which intergrade more or less, in a progressive way, as 

 one passes down stream from the head-waters ; one extreme 

 type being in the head-waters and the other farther down stream. 

 It is the degree of continuity or progressiveness in the deviation 

 of the characters which clearly points in definite directions, and 

 emphasizes the necessity of a study of series taken along the 

 lines of dispersal. 



In the case of the land fauna the situation is much more com- 

 plex, yet if highways of dispersal are considered analogous to 



1 Proc. A. A. A. S., 1900. 



