No. 634] STKXOTIIKBMY AXD ZOXE-IXYASIOX 



nature of the fundamental differences between the eury- 

 thermal and the stenothermal species, and this, in turn, 

 is closely connected, as I shall hope to make plain, with 

 the second topic of this paper, viz., zone-invasion. 



I have already tried to make clear the fact that my in- 

 vestigations have tended very strongly to convince me 

 that each and every species of marine plant is normal 

 to only one zone, and that, when a species is credited to, 

 or found to occur in, two zones, it is normal to only one 

 of them and is to be found in the other because for some 

 reason it finds in the second zone the temperature con- 

 ditions, both as to degree of temperature and as to dura- 

 tion of that degree of temperature, of the zone to which 

 it is normal. In a similar fashion, if a species is found 

 to inhabit three, four, or even five zones of different tem- 

 perature relations, it is possible to make certain that it 

 is normal to only one of these and invades the other 

 zones because it finds the proper temperature conditions 

 for its continuous existence. The proper temperature 

 seems certainly to be that which is most intimately con- 

 nected with reproduction, since it is this function that is 

 most necessary to persistence in the particular locality. 

 In the laboratory, under controlled conditions, alga 1 , in 

 particular, have been found to be very sensitive to even 

 slight changes of temperature, as Kwart (18! Hi) has dem- 

 onstrated. It does not seem as if the same algae, in their 

 ordinary environment, could be thus sensitive as West 

 and West (1898) have held, but Ewart (1898) has an- 

 swered their objections, claiming that, in nature, they 

 are probably equally sensitive, but withstand seemingly 

 great changes for reasons that prevent these changes 

 actuating. This is something of the truth in the case of 

 species invading colder from warmer, or warmer from 

 colder zones in that they find in the invaded zones the 

 temperatures, both as to intensity and duration, which 

 are favorable to their growth and reproduction and 



zone. 



