256 



SCIENCE. 



[N. S. Vol. XXIII. No. 581. 



with the typical form, but replacing it in 

 the territory between it and that occupied 

 by the new foria, so that as a rule all the 

 individuals from the transitional territory 

 are intergrades. This fundamental fact' 

 in geographic biology seems to have escaped 

 the keen eye of Darwin, who, in speaking 



The only explanation I think of to account 

 for Darwin's and Morgan's failure to 

 recognize this fact is limited or unfavor- 

 able field experience, and limited experi- 

 ence in handling specimens from the per- 

 ipheries of geographic areas large enough 

 to cover the transition from one faunal 



3. Distribution of the Prairie Ground Squirrel, 



species. 



Citellus tridecemlineatiis and sub- 



of the difficulties and obstacles in the way 

 of his theories, said : 



"Why, if species have descended from 

 other species by fine gradations, do we not 

 everywhere see innumerable transitional 

 forms? Why is not all nature in confu- 

 sion, instead of the species being as we see 

 them, well defined?" One of his critics, 

 Thomas Hunt Morgan, thinks this a very 

 serious matter. My reply to both Darwin 

 and Morgan is that they are mistaken in 

 their premises, for in nature transitional 

 forms are so abundant as to be a source of 

 annoyance and embarrassment to system- 

 atists and museum curators — men who are 

 continually handling specimens which they 

 desire to refer to one .species or another. 



division into another— in other words, fail- 

 ^^re to study the behavior of species in 

 nature from the geographic standpoint. 

 One might spend a lifetime in studying 

 animals and plants in the interior of al- 

 most any of the faunal areas without en- 

 countering transitional forms or inter- 

 grades, for it is at the peripheries or bor- 

 ders of these areas that the intergrades 

 occvir. It would seem, therefore, that Dar- 

 win, in his effort to present fairly all ob- 

 jections to his theory, imagined a difficulty 

 which does not exist, and that his critic 

 Morgan solemnly shook his head at a man 

 of straw. 



Conclusion.— Inasmweh as all species 

 have their beginnings in variations, and 



