DISTRIBUTION OF MARINE ALQM. 165 



Struvea is commonly found at considerable depths, one of 

 the species at more than thirty fathoms. Besides these 

 extreme cases there are many others not so marked in 

 character, which modify this zonal distribution, but the 

 fact remains generally true of much the larger proportion 

 of forms. On what grounds are we to account for this 

 regulation of pigment ? We know that sea-water inter- 

 cepts sunlight and that the deeper we go the darker it 

 becomes — that at a depth of 700 fathoms, probably less, 

 there is total darkness, and that fishes which live at great 

 depths are blind, like those of the Kentucky caves. 



If we apply this knowledge to our problem, the observa- 

 tion will not escape us that the forms living close inshore 

 and most exposed to sunlight are green, as has just been 

 noticed, that they are of the same colour as land vegeta- 

 tion and fresh water vegetation, the illumination being 

 similar. Now though we know very little about the 

 interception of sunlight by sea water, yet this much is 

 certain, that not only is the quantity of light reduced, but 

 its quality is also affected, as spectroscopic investigation 

 has shown. It is precisely those rays that are most 

 efficient in the work of assimilation by plants that are first 

 intercepted, and only the green and blue rays travel far. 

 This change of pigment we may then fairly take it, is an 

 adjustment to the supply of sunlight, and if we enquire 

 farther we shall find that the essential green pigment — 

 the chlorophyll on which we know the nutrition of plants 

 depends — is only masked by the red (phyco-erythrine) 

 brown (phyco-phseine), blue (phyco-cyanine) , and yellow 

 (phyco-xanthine) pigments to be found in those seaweeds 

 near and beyond low water mark. There is very little 

 known as to the precise functions performed by these 

 modifications of chlorophyll, whether their office is to 

 heighten the susceptibility of the chlorophyll to sunlight 



