THE AMERICAN NATURALIST [Vol. LIV 



another of these zones, that a considerable number of 

 species extend over two of these zones, that a compara- 

 tively small number are found to extend over three zones, 

 while the number credited with extending over four or 

 five zones are extremely few and almost always doubt- 

 fully so accredited. It was also suggested that the dis- 

 turbance of zonal distribution, so far as the occurrence 

 is concerned, is probably due to spot distribution, i.e., 

 where waters of a higher or lower temperature than that 

 of the zone in which they are placed exist due to local 

 physical conditions, and to seasonal lowering of the 

 temperature normal to the zone. In 1916, in another ad- 

 dress (published 1917), I reasserted these statements 

 and added something as to the significance of the isocry- 

 mal lines, or lines of monthly mean minima. I suggest 

 using the latter lines to divide the zones into proper 

 provinces. 



Since writing the last paper I have investigated the 

 floras of the coast of Xew England and have found that 

 the species may be readily arranged in two categories, 

 one of the colder waters (20° C. or less) and the other of 

 the warmer waters (20° C. or over), and while some of 

 these are found only north of Cape Cod and others only 

 south of Capo Cod, the majority are found on both sides 

 of the cape, which is, however, the natural dividing point 

 and approximating closely to the position of the 20° C. 

 isothere. The separation is made by ascertaining 

 whether a given species of the last group in p;i rlicii lar 

 inhabits warmer localities to the north or is found only 

 in cold localities or appears or fruits only in the colder 

 season to the south. Similar examinations of other, but 

 less perfectly known floras, add to the conviction that 

 species of marine algae, at least, are normal to only one 



the cases of those 1 of the very coldest waters and itmay 

 be that they are no exceptions to such a rule. Further- 

 more, it may be assumed, from observing the isotheres 

 and isocrymes in favorable portions of the surface waters 

 of the oceans, viz., those undisturbed by the larger ocean 



