No. 634] STENOTHEEMT AND ZONE-INVASION 389 



currents, that the normal, or at least the minimum sea- 

 sonal variation in temperature is closely approximating 

 to 5° C. This, added to the amplitude of 5° C. mean 

 maximum variation, makes the normal amplitude of tem- 

 perature within each zone about 10° C. and the tempera- 

 ture interval favorable to the persistence of a species 

 within a given area, so far as active growth is concerned, 

 is very little, if any, over 10° C. The seasonal range in 

 some portions of the surface waters of the oceans may 

 amount to as much as 18° or 20° C. In such localities as 

 may have such an extreme range of temperature, we may, 

 I think, assume that a condition of quiescence, or rigor, 

 may exist, at least in the perennial species, such as exists 

 in the case of perennial plants in zones on land where 

 there is an alternation of a frost with a frostless season, 

 as it does particularly in the polar and most of the so- 

 called temperate regions. 



It will appear from a careful consideration of what I 

 have been saying, that the temperatures for normal per- 

 sistence of any particular species of marine plant lie 

 within narrow limits, although many marine plants are 

 credited with extending over fairly wide ranges of tem- 

 perature. We are, consequently, brought directly to a 

 consideration of the ideas implied in the use of the terms 

 stenothermal and eurythermal. The proposal of these 

 terms, or rather their equivalents in German, rests with 

 Karl Moebius who, in 1877, published a paper in Die 

 Natur on the external factors of life of marine animals. 

 According to Moebius the eurythermal animals can en- 

 dure wide ranges of temperature and continue their 

 occupation of extensive zones and range in depth be- 

 cause they are able to reproduce under such conditions. 

 It is this conception of being able to reproduce at widely 

 separated temperature limits that I wish to call particu- 

 lar attention in order that I may discuss it later. Moe- 

 bius states that the eurythermal animals are much less 



