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THE POPULAR SCIENCE MONTHLY. 



mal. Each of these families of the great invertebrate kingdom has 

 been bandied back and forth from the botanist to the zoologist, and 

 each has finally found its place in the animal world. 



No purely empirical knowledge is sufficient to determine, among 

 the lower forms of life, to which kingdom they should be referred. It 

 is only by studying facts in their relations, by patiently observing the 

 life-history, and by ascertaining the modes of nutrition and reproduc- 

 tion of each form, that its true place in the organic world has been 

 determined. 



Pig. 2. Diageam op a Section of Htdroid. 



It was, for many years, thought that, beyond the depth of 300 

 fathoms, organic life ceased to exist in the ocean. Forbes reached this 

 zero of life in the .^Egean Sea, and the fact ascertained for the Medi- 

 terranean was inferred for all other seas. The transmutation of inor- 

 ganic into organic matter is only performed by vegetables, and then 

 only under the controlling power of light. The distinction made by 

 naturalists between the lowest forms of animal and vegetable life lies 

 just here : vegetables convert the inorganic elements of earth, air, 

 and water, into organized matter; animals rearrange this organized 

 matter into animal tissue. It is well known, as no light penetrates 



