INTRODUCTION 7 



As light is the factor that determines the zonal 

 distribution of seaweeds, and thus influences their 

 local habitats, so temperature is the leading in- 

 fluence, among others of minor potency, that affects 

 their geographical distribution. They inhabit a 

 medium of stable temperature, in which they are 

 not called upon to adjust themselves to any great 

 periodical or fortuitous changes, varying little from 

 day to night and from season to season. On this 

 ground alone they might be presumed to be pecu- 

 liarly sensitive to change of temperature, and experi- 

 ments in the culture of seaweeds in aquaria show 

 that a nice regulation of temperature is necessary to 

 success. Comparisons of the marine flora of areas of 

 different temperature confirm this view though they 

 do not exclude other possibilities, since the general 

 character of such floras is modified by other in- 

 fluences the nature of the bottom to some extent, the 

 degree of salinity of the water, the presence, absence 

 and amount of the tides, &c. Marine vegetation, like 

 fresh-water vegetation, is removed from the influence 

 of relative humidity which determines frequently the 

 character of a land flora, but on the other hand it is 

 subject to control by factors such as relative salinity 

 and the like. The contour of the earth's surface, 

 which brings about the existence of alpine floras for 

 example, and frequently affords many climates at the 

 same latitude on land, has no corresponding 

 influence on the marine flora, since conditions of 

 illumination check range in depth. On the whole, 

 temperature may be said to be left more to itself 

 as a determining influence of the character of marine 



