ON THE RELATION BETWEEN, ETC. 371 



mena — metabolism and organic formation. For example, 

 metabolism is practically inconceivable without a fluid 

 condition of the substance undergoing change, while, on 

 the other hand, the idea of a definite form involuntarily 

 suggests a fixed position of the particles of matter of which 

 the organism is composed. This difficulty, however, is 

 merely superficial, and vanishes as soon as we examine 

 the two seemingly irreconcilable facts, A fluid stratum is 

 certainly a requisite of metabolism. In the words of the 

 ancient alchemists, coj'pora non agunt nisi humida. Only 

 gases or dissolved matter can enter into the chemical 

 relations required in the metabolism of an organism. 



This truth ought not to be ignored, as it has been by 

 those who have maintained that the cell substance is more 

 or less compact and solid. Many have regarded the pro- 

 toplasm of a cell, including its nucleus, as a sponge-like 

 network, others, merely recently, as a thread-like structure, 

 composed of numberless fibres. To Berthold and Butschli, 

 however, belongs the credit of strongly insisting upon the 

 fluid nature of protoplasm. Those who hold the opposite 

 view base it solely on the examination of dead and pre- 

 served objects ; it is difficult to imagine how any one can 

 maintain it who has studied the life processes in living cells, 

 e.g., the protoplasmic movement in a creeping Amoeba, the 

 many-branched system of currents in a Myxomycete plas- 

 modium, or the rotation of the protoplasm in a plant cell. 

 On the other hand, there can be no doubt that protoplasm, 

 although essentially a fluid, does contain some more solid 

 and firm elements, i.e., it is a compound of substances of 

 various consistencies. 



When we find ourselves face to face with the question, 

 " What are we to consider as actually living in living 

 matter ? " we have but one criterion to assist us to a 

 decision, and that is metabolism. Only where there is 

 metabolism is there life. This is the A B C of physi- 

 ology. Hence, we can only call those particles alive which 

 are undergoing chemical changes. We must consider as 

 not alive, in the most restricted sense of the word, all matter 

 which is not at the moment chemically active, even though, 



