170 University of California Publications. IBotant 



very different from the coast of North America to the south. In- 

 stead of a bold and even coast, with few deep harbors and few 

 islands, the Northwest Coast is much indented and bordered by 

 islands, or in fact archipelagoes, throughout much of its extent. 

 This is significant when we are considering the question of dis- 

 tribution, for the greater portion of the collecting has been done 

 in the inner and more sheltered portions of the coast, and may 

 account for the absence of some species considered characteristic 

 of the next lower region. 



The temperatures of the surface waters are of extreme inter- 

 est to us, because they are the principal factors concerned in 

 limiting distribution and bringing about the existence of general 

 geographical regions of distribution. 



One of us (cf. Setchell, 1893) has already shown that a change 

 in the kelp-flora takes place with the increase of every five de- 

 grees ( C . ) of surface temperature . xhe present idea of regional dis- 

 tribution on the western coast of North America bears out the 

 same idea. The statement is crude, because the data are not ex- 

 tensive and exact enough. The average difference between the 

 maximum and minimum of temperature for a given region is in 

 the neighborhood of five degrees also. While the isotheres and 

 isocrymes can be plotted only roughly at present, we find reason 

 for believing that the isoeryme of 5° C. and the isothere of 10° C. 

 pass just below the Aleutian Islands or about the southern 

 limit of the Upper Boreal Region; that the isoeryme of 10° C. 

 and the isothere of 15° C. pass through the Strait of Juan de 

 Fuca or somewhere near there and correspond nearly to the 

 southern limit of the Lower Boreal Region ; that the isoeryme of 

 15° C. and the isothere of 20° C. pass through Point Conception 

 or near it and correspond nearly with the southern limit of the 

 North Temperate Region of our coast; and that the isoeryme of 

 20°C. and the isothere of 25° C. pass near Cape San Lucas, or 

 near the southern limit of our Subtropical Region. Although there 

 is need that these isothermal lines be determined as to position on 

 our coast with much greater accuracy, yet the coincidence of the 

 lines mentioned as determined by the temperature of the surface 

 waters and the limits of the geographical regions determined by 

 a study of the distribution of the species of alga?, is certainly 



