92 SECRETS OF ANIMAL LIFE 
tive chain, the pound of flesh will be, so to speak, 
cheaper. Thus a pound of plaice is said to require 
to begin with only 100 lb. of vegetable material. 
But the basal fact is clear that just as all flesh is 
“orass,”’ so sooner or later all fish is “ seaweed.” 
In spite of the old saying, Vittor alga,—“ More 
worthless than seaweed,’’—these humble plants have 
many uses, ¢.g. in making mannite, mucilage, and 
manure. The nutritive value of seaweed is, indeed, 
an old story; the streets of Edinburgh used to 
resound with the shrill cry of the fishwives— 
“Wha’ll buy dulse and tangle?”’ but what science 
has shown is that the indirect importance is much 
greater than the direct. 
It is plain, then, that fishes are far from being 
economical to produce. They are like a super- 
structure that requires a very broad and costly 
foundation. The quantity of humbler life that 
makes the food-fishes—almost wholly carnivorous— 
possible is enormous; and the estimate has to be 
increased when we remember that a great proportion 
of the weight of an animal which a fish devours 
may be quite useless—e.g. the water in its tissues 
and the shell of lime. Thus we are naturally led to 
Professor Petersen’s important practical conclusion 
that the quantity of fishes which an area of sea 
can support is anything but unlimited. In some 
restricted bays, indeed, he found strong reasons for 
suspecting that the limit had been reached. That 
this limit is a generous one is an important fact for 
man, for the Danish fishermen took about 60,000 
