HOW PLANTS AND ANIMALS GROW. 505 



sented the essential constituents of the cell and active elements in 

 plant nutrition. 



The identity in essential features of Dujardin's sarcode and 

 Von Mohl's protoplasm was pointed out by Cohn in 1850, and 

 fully demonstrated by Max Schultze in 1858, who, adopting the 

 term protoplasm, defined the cell as "a unit-mass of nucleated 

 protoplasm, with or without a cell wall," and vegetable and ani- 

 mal physiology were thus placed on a common correlated basis. 

 The original cell theory was materially modified and, in fact, su- 

 perseded by the conception that the units of organized structure 

 are masses of protoplasm, more or less intimately related, from 

 and by which organic matters, including the cells and various tis- 

 sues, were formed. 



In 1868 Prof. Huxley translated protoplasm into the significant 

 phrase, " the physical basis of life," and all vital activities were 

 assumed to be the result of its inherent properties. While ad- 

 mitting the general pertinence of this assumption, we should not 

 fail to notice that many of the inferences from the facts then 

 known have not been verified in the progress of knowledge, and 

 recent investigations have materially modified our views as to the 

 real composition and constitution of protoplasm. 



From what was known in regard to protoplasm twenty years 

 ago, it appeared to be reasonable to assume with Huxley that 

 there is " one kind of matter which is common to all living beings, 

 and that their endless diversities are bound together by a phys- 

 ical as well as an ideal unity " ; that vegetable and animal proto- 

 plasm are strictly identical ; that " an animal can not make proto- 

 plasm, but must take it ready made from some other animal or 

 some plant " ; or, in other words, that the protoplasm made by 

 plants from mineral matters is, in fact, the physical basis of ani- 

 mal life. 



At the present time we may look upon protoplasm as the phys- 

 ical basis of life in the sense that some form of it is the essential 

 and active constituent of every living cell or tissue, whether vege- 

 table or animal, and that it is only formed through the physio- 

 logical activities of living organisms. In the absence of life, pro- 

 toplasm can not be formed, and, so far as we can perceive, there are 

 no manifestations of life without it ; but we can no longer assume 

 that it is a substance of the same chemical composition and con- 

 stitution in all the varied conditions under which it appears in 

 the different groups of plants and animals, or even in the differ- 

 ent organs of the same individual. Protoplasm is a convenient 

 name for living substance, but we must bear in mind that it is the 

 most complex and unstable of organic substances, and varies 

 widely in structure, specific properties, and probably in chemical 

 composition. 



