INTRODUCTION 5 
the sea that an explanation of their nature must be 
sought. It has been found that the colours of sea- 
weeds are more or less indicative of their range in 
depth in the sea, and, allowing for numerous excep- 
tions, that there is a zonal distribution of Algz 
according to their colours. The uniformity of this 
distribution is disturbed by the fact that the con- 
ditions are not equal for all in the face of the deter 
mining influence, as will presently be made plain 
Asa general rule the inshore seaweeds near high- 
water mark are green in colour like land vegetation 
and lower down between tide-marks there is a belt 
of olive forms sheltering red plants beneath them. 
Where rocks overhang the bottom, and in small pools 
these red forms also occur at this level. At extreme 
low-water mark, and beyond it, are found the brown 
tangles sheltering red forms again, while at the lowest 
depths of plant-life in the sea the red forms occur 
without shelter. Between 20 to 50 fathoms seaweeds 
become more and more rare, while below that depth 
their occurrence is exceptional. That the main 
influence determining this regulation of pigment is 
the nature of the supply of sunlight, necessary to the 
action of chlorophyll in the work of nutrition, is 
apparent from the following facts. The interception 
of sunlight by sea-water brings about a state of total 
darkness at 700 fathoms, probably less, and though 
seaweeds do not penetrate to a depth approaching 
this limit of light, a further consideration will account 
for their failure. Not only is the quantity of sun- 
light reduced by its passage through the water, but 
its quality is affected, as spectroscopic investigation 
