No. 504] 



NOTES AND LITERATURE 



377 



other centers of dispersal wherever there are branches in the 

 forward movement of the group; but I understand the usual 

 meaning of "center of dispersal'* to be the point or points from 



spread of the group. 



2. Location of Dominance or Gnat Abundance of Individuals. 

 — This criterion seems futile. A group may have moved in all 

 directions from a given region and died out entirely in the re- 

 gion from which it moved. Nothing is more clearly established 

 than the changeableness of geologic climate and hence of all en- 

 vironmental factors and we know of many instances where noth- 

 ing is left of a group of organisms but fossil remains in the re- 

 gions of their former abundance and a few living remnants in 

 far-away, protected spots. If, as seems very clear, we can not be- 

 lieve that even the most populous of these havens marks the cen- 

 ter of dispersal of such a group, neither can we apply this cri- 

 terion with safety to any other group. The area of present domi- 

 nance is merely that area, of all those now inhabited by a group, 

 which is at the present time most suited to the group — unless, of 

 course, it has arrived in a more suitable area so recently that it 

 has not had time to develop its dominance. 



3. Location of Synthetic or Closely Belated Forms.— This cri- 

 terion will be considered more in detail later, but it may be re- 

 marked in passing that the location of closely related forms is of 

 little help in arriving at the center of dispersal unless we know 

 whether these forms are more primitive or the reverse and un- 

 less, furthermore, we liave selected the right one of the two oppos- 

 ing theories which were mentioned in the beginning of this dis- 



4. Location of Maximum Size of Individuals.— It is difficult to 

 see why individuals should be larger at the center of dispersal 



to present-day, optimum environmental conditions, but this is 

 probably not often, and certainly not necessarily, anywhere near 

 the ancient center of dispersal. 



bility, in Crops.— Adam says this criterion is very closely re- 

 lated to the second one. If so, it fails for the same reason. Also, 

 it would seem that the latter part of this criterion conflicts with 



6. Continuity and ~ColvlfgeLe of Lines of Disposal— This 

 certainly ought to work, provided we follow the lines in the right 

 direction. The difficulty is that north and south lines on a more 



