IO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



live, it is economically much more specialized in its inherent ability 

 to exist over a very wide range. Though much more specialized than 

 the original type, this new dominant type will also be much less spec- 

 ialized than many of the types which it supplants, which will have 

 possessed a very high degree of specialization in order to meet very 

 highly specialized conditions. 



The sum of the effect of this organic progress may be expressed by 

 the statement that any animal type, once evolved, will extend itself 

 immediately in every direction as far as the natural barriers to its 

 dispersal; a more specialized form (a dominant type) of the same 

 animal, better fitted for the conditions under which it lives, will sooner 

 or later be evolved somewhere in the central, or most favorable, 

 portion of the territory inhabited by the original type ; this new type 

 will at once extend itself as did the original type ; but in. the meantime 

 there may have arisen certain barriers which the second type cannot 

 cross and beyond which, therefore, the first type is secure. Up to 

 these barriers — high mountains, deserts, newly formed arms of the 

 sea, or whatever they may be — the second type will gradually sup- 

 plant the first, as a result of its better economic equipment and more 

 perfect physical resistence, and the advantages which this better 

 equipment and resistence give it in the struggle for existence. Thus 

 we shall eventually find a specialized type beyond the limits of which 

 occurs a more generalized type of the same organism. The sub- 

 sequent evolution of additional types, which will most frequently 

 occur at or near the so-called center of distribution as a natural result 

 of the greater facility for adaptation due to the greater distance apart 

 of the physico-economic barriers and the consequently greater radius 

 of each type, will result in the gradual formation of a dispersal figure 

 which would be ideally represented by a series of concentric circles, 

 each of the circles representing a barrier, the small central circle en- 

 closing the most perfected type and the peripheral band the most gen- 

 eralized, the intervening areas including intermediate types increasing" 

 in specialization toward the center. 



The distribution of the Peripatidse represents a sector of such an 

 ideal dispersal figure ; the center of distribution for the family is the 

 Malayan region, where the most specialized type occurs ; just west of 

 this is the great barrier of the Indian Ocean ; in central Africa we find 

 a less specialized type which probably reached its present habitat 

 long before the type now occurring in the Malayan region was 

 evolved, and which has been protected from the encroachment of that 

 type by the submergence of the land over which it originally migrated. 



