390 THE AMERICAN NATURALIST [Vol. 



usual type and this agrees with what I have found in my 

 attempts to tabulate the marine algae as I have already 

 mentioned with emphasis. To repeat the idea of Moe- 

 bius, in a rather free translation of his own words, eury- 

 thermal animals are those which in the surface waters 

 of the temperate zones are able to exist and to continue 

 their kind through reproduction under all of the various 

 temperature relationships of the different seasons of the 

 year. 



The terms eurythermal and stenothermal have not 

 come into any noticeable use in botany and are not wide- 

 spread even in zoological literature, although they are 

 very convenient. They are both to be found in the later 

 editions of Webster's and in the supplement to the Cen- 

 tury Dictionary. They are discussed in the latest edi- 

 tion of the Encyclopedia Britannica by Gr. H. Fowler, 

 under the article on "Plankton." Fowler says: "In 

 relation to temperature the wide-ranging species are 

 termed eurythermal, the limited stenothermal (Moe- 

 bius) ; the terms are useful to record fact, but not ex- 

 planatory. It seems to be the case that to every or- 

 ganism is assigned a minimum temperature below which 

 it dies, a maximum temperature above which it dies and 

 an optimum temperature at which it thrives best; but 

 these have to be studied separately for every species." 

 The definitions of Moebius and the comments of Fowler 

 are exactly to the purpose of our consideration, since the 

 one is from the purely distributional point of view ex- 

 pressing a fact only, while the other seeks to link the 

 fact with some explanation, preferably physiological. 

 Our own discussion of these terms and the underlying 

 conceptions must necessarily proceed on a somewhat 

 middle course, largely from the distributional point of 

 view, but with such regard for the interpretation of the 

 physiological basis as may be possible from our present 

 knowledge. 



It will be of the greatest assistance, I think, to consider 

 some concrete cases of eurythermal species and to in- 

 quire into the conditions of their continued persistence 



