The Zoologist — November, 1873. 3743 



water ; the same sea must be used again and again ; the same water 

 must be encompassed by the same land. However clearly the 

 theory of change in relative position may have been established — 

 however manifest it may be that the ocean now rolls over what was 

 once dry land, and that what is now dry land was once covered by 

 the sea — still there is nothing new, nothing added, nothing sub- 

 tracted ; the same materials remain, fluid and solid, and will remain 

 for ever. Motion continually exposes a fresh surface to the atmo- 

 sphere, and this contact of air and water, called aeration, is in 

 continual operation. 



*' The great want felt," says Mr. Lloyd, " was constant motion like that of 

 Nature, by which the water may be continually turned over and over, 

 presenting successive and multitudinous surfaces to the surrounding atmo- 

 sphere, and may by contact with it incessantly absorb large quantities of 

 oxygen necessary for the sufficiently rapid decomposition of organic matter 

 given off by the animals." — 'Handbook,' p. 18. 



Twenty years before this the absolute necessity for motion 

 and aeration was, I believe, first pointed out by Mr. E. W. H. 

 Holdsworth, in his ' Handbook to the Fish House in the Gardens 

 of the Zoological Society of London.' This excellent little pam- 

 phlet is not only useful as a companion to the aquarium, but is a 

 philosophical exponent of the only principles and arrangements on 

 which marine aquariums can be established so as to become per- 

 manently successful. Mr. Holdsworth has shown himself perfectly 

 acquainted with his subject, and thoroughly competent to explain 

 it to others. The 'Handbook' is now so exceedingly rare that I 

 need make no apology for the rather long quotation given below. 



" The main difficulties at present met with in the satisfactory maintenance 

 of the aquarium are unquestionably due to our ordinary inability to imitate 

 that most important condition of the sea — its continual motion. The 

 advantages derived from that movement are incalculably great to its 

 inhabitants ; so much so as to mask, in a great measure, the principle of 

 compensation on which plants and animals are largely dependent for the 

 supply of the gases necessary for their respiration. In a small tank the 

 presence of decaying animal or vegetable matter, even in inconsiderable 

 quantities, is often sufficient to destroy the purity of the water, and so to 

 cause the death of the animals in it ; for the poisonous gas, carburetted 

 hydrogen, arising from putrefaction, is there confined within a small space, 

 and the stagnant condition of the water prevents its rapid combination with 

 the oxygen, which, uniting with the hydrogen, produces water, and with 

 the carbon results in carbonic acid. But, in the sea, any dead matter not 



