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THE AMERICAN NATURALIST [Vol. XL VII 



E. Species increasing in relative abundance as the depth increases, 



reaching a maximum at a depth of 250 fathoms or more F. 



EE. Species increasing in relative abundance as the depth increases, 



but reaching a maximum between 150 and 250 fathoms G. 



F. Species of relatively common occurrence above 150 f athoms . . S. lyra. 

 FF. Species whose occurrence above 150 fathoms is exceedingly 



rare S. planktonis. 



G. Species never occurring above 50 fathoms K.subtilis. 



GG. Species never occurring above 25 fathoms E.hamata. 



GGG. Species occurring at irregular times above 25 fathoms, and some- 

 times even on the surface S. serratodentata. 



It is unnecessary to state that this key is not published 

 for the purpose of furnishing a ready means of identi- 

 fying the various species of Chaetognatha. Perhaps, 

 when all the species from the four quarters of the globe 

 have been studied as critically in regard to their behavior 

 and ecological relations as they have in regard to their 

 morphology, it will be possible to construct a ready means 

 of identification on such a basis, but at present we can do 

 no more than point out that the key does work for the San 

 Diego region and ascertain what this fact signifies. 



Its primary significance is that species are quite as dis- 

 tinguishable from their manner of distribution as from 

 their morphological characteristics. In other words, each 

 species has its own definite and distinctive mode of 

 behavior and each adapts itself to the hydrographic and 

 other elements of its environment in quite as definite a 

 way as any of the other species. 



This being true, the question at once arises: To what 

 extent are morphological differences between the species 

 proportional to, or correctable with, their distributional 

 differences. Eitter ('09) has pointed out that, if 

 " change of environment and of environed organism are 

 wholly and inseparably linked together," one ought to be 

 able to measure and correlate the differentials between 

 organisms with the differentials between their environ- 

 ments. However, in attempting to find such a ' ' necessary 

 correlation" in the case of Halocynthia johnsoni, native 

 to the San Diego region, and H. Jianster, native to the 

 Washington coast, the results were negative. It is un- 



