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 Algae 

 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 

 As a 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 



