ISOLATION AND SELECTION IN THE EVOLU- 

 TION OF SPECIES. THE NEED OF 

 CLEAR DEFINITIONS 



JOHN T. GULICK 



The discussion concerning the factors in organic evo- 

 lution has been obscured by the diversity of meanings 

 given to the same terms by different writers, and some- 

 times by the same writer. What do we mean by isolation, 

 by selection, by environment, by evolution? 



Different Meanings given to Isolation 

 I think that in Darwin's books isolation is always used 

 to designate the prevention of free-crossing between dif- 

 ferent groups by means of factors lying outside of the 

 groups, such as geographical barriers, but it has since 

 been extended to mean the prevention of free-crossing by 

 any means. Professor Kofoid, in his article in Science 

 for March 29, 11)07, gives the word this broader meaning 

 in one place; but he must use it in a more limited way, 

 when in the last sentence of his article, speaking of what 

 DeVries finds in the elementary species "coincidently 

 appearing" in the evening primrose, he says: " Isolation 

 plays no part in their origin or continuance. ' ' I have not 

 found any statement by DeVries maintaining that ele- 

 mentary species remain distinct forms from the original 

 stock, when free-crossing with the original stock contin- 

 ues. In my volume on "Evolution, Racial and Habitu- 

 dinal,"pp. 5, 69-70, 155-156, 1 call attention to a mutation 

 arising m certain species of snails, and probably pro- 

 ducing complete isolation between the new form and the 

 old, though both are occupying the same tree. The her- 

 maphrodite snail is so constructed that it seems impos- 

 sible for one of sinistral form to cross with one coiled in 

 the opposite way; but in the Hawaiian Islands there are 



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