NOTES AND LITERATUEE 



FAUNAL DISPERSAL 1 



The widely different conclusions drawn by different students 

 from the same facts is well illustrated by Matthew's recent paper 

 on the geographic distribution of animals. 5 Although these dif- 

 ferences are of many and varied sorts a fundamental one seems 

 to be concerned with the question as to whether the peripheral 

 part or the central part of the range of a group contains the 

 more progressive members of that group. 



The idea which is held, either consciously or unconsciously, by 

 many is that after a group has arisen it spreads; and the mi- 

 grants, meeting new conditions, develop new characters. These 

 new forms then spread still further and develop other charac- 

 ters. Thus the more primitive members remain at or near the 

 point of origin of the group ; the successively more progressive 

 members will be found at respectively greater distances from 

 this point. According to this theory we may trace backward the 

 dispersal of the group by following the distributions of the sev- 

 eral members from the most progressive to the most primitive. 

 We may not be able to follow this line to the actual point of 

 origin of the group for the most primitive members may not be 

 alive now and we may be unable to find their fossil remains. 

 However, as organisms usually extend their range in more than 

 one direction, we may be able to trace several lines of dispersal 

 and deduce that the point of origin was near the place at which 

 these lines tend to converge. 



The other theory is as nearly the opposite as can be.' Accord- 

 ing to it the first progressive makes its appearance well within 

 the range of the primitive members of the group. If the new 

 form is not an improvement over the primitive one, it dies out. 

 If it be an improvement, it crowds the primitive form which is 

 forced to leave its place of origin and migrate. It is sometimes 

 made a part of this theory (but it is not a necessary part) that 

 the reason a new form appeared among the old was that the new 

 climatic conditions appeared there and that if the old form could, 

 in its migration, keep in climatic conditions suited to it, well and 

 good; if it could not, it either died out or became adapted to the 



*Bead at the February 15, 1916, meeting of the X Y. Entomological 

 Society. 



2 W. D. Matthew, 1915, "Climate and Evolution," Annals N. Y. Acad, 

 of Sciences, XXIV, pp. 171-318. 



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