No. 594] NOTES AND LITERATURE 



375 



climatic conditions into which it was forced without, however, 

 losing all the earmarks of its primitiveness. Later still, newer 

 forms appeared at the old stand and forced their predecessors 

 still further away. Therefore, if we follow the distribution of 

 the several members of the group from the most progressive to 

 the most primitive, we will trace the dispersal of the group 

 forward, not backward. 



It would probably be impossible to decide, either by logomachic 

 methods or by watching present-day movements of species, whirii 

 of these two theories is correct. However, we are not left with- 

 out hope because paleontologists are daily digging up evidence 

 which, when sufficiently complete and properly translated, will 

 leave us in no doubt as to the history of the dispersal of certain 

 groups, and there is little doubt that the same general principles 

 which hold for those groups will apply to others concerning 

 which we have not and probably never will have fossil record. 

 Of course details may differ from group to group, but it is not 

 probable that there is one set of laws for mammals and another 

 for reptiles, one for birds and another for insects, that nature 

 constructed a trans-oceanic bridge for one group, but stationed a 

 guardian angel on it to prevent the passage of others. The chief 

 differences probably were such as the differing responses of the 

 different groups to changes of the environment and the differing 

 powers of different groups in overcoming given barriers to dis- 

 persal. 



Next to that brought about by the holding of the diametrically 

 opposing theories just discussed, perhaps the most important 

 source of confusion is in not keeping clearly distinct " center of 

 origin," "center of dispersal" and "center of greatest develop- 

 ment." They may all be in the same region or they may be as 

 far removed from each other as the earth's surface will permit 

 but they are not the same. A group may arise at A and move to 

 B, from which point there are easy paths of migration to C, D, 

 E and F. It may, and doubtless would, go to all of these but it 

 might find that E alone furnishes good conditions for its future 

 development. Its center of origin would then be at A, its cen- 

 ter of dispersal would be at B, and its center of greatest develop- 

 ment at E. According to the first theory discussed above, the 

 more primitive forms would be found at A or B and the later 

 developments at C, D, E and F, but chiefly at E. According to 

 the second theory, C, D, E and F would have the primitive forms 

 (although they might be much modified, especially at E) and the 

 progressive members of the group would be at A or B. 



