INTRODUCTION vi 
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 
