LIVING MATTER. 5 



plasm in virtue of which they break up into more highly oxidated products, 

 which cease to form any part of the living body, is a constant concomitant 

 of life. There is reason to believe that carbonic acid is always one of these 

 waste products, while the others contain the remainder of the carbon, the 

 nitrogen, the hydrogen, and the other elements which may enter into the 

 composition of the protoplasm. 



" The new matter taken in to make good this constant loss is either a 

 ready-formed protoplasmic material, supplied by some other living being, 

 or it consists of the elements of protoplasm, united together in simpler 

 combinations, which constantly have to be built up into protoplasm by the 

 agency of the living matter itself. In either case, the addition of molecules 

 to those which already existed takes place, not at the surface of the living 

 mass, but by interposition between the existing molecules of the latter. If 

 the processes of disintegration and of reconstruction which characterize 

 life balance one another, the size of the mass of living matter remains sta- 

 tionary, while if the reconstructive process is the more rapid, the living 

 body grows. But the increase of size which constitutes growth is the 

 result of a process of molecular intussusception, and therefore differs alto- 

 gether from the process of growth by accretion, which may be observed in 

 crystals, and is effected purely by the external addition of new matter ; so 

 that, in the well-known aphorism of Linnaeus, the word ' grow ' as applied 

 to stones signifies a totally different process from what is called ' growth ' 

 in plants and animals. 



" 3. Its tendency to undergo cyclical changes. In the ordinary course 

 of nature, all living matter proceeds from pre-existing living matter, a 

 portion of the latter being detached and acquiring an independent exist- 

 ence. The new form takes on the characters of that from which it arose ; 

 exhibits the same power of propagating itself by means of an offshoot ; 

 and, sooner or later, like its predecessor, ceases to live, and is resolved 

 into more highly oxidated compounds of its elements. 



" Thus an individual living body is not only constantly changing its 

 substance, but its size and form are undergoing continual modifications, 

 the end of which is the death and decay of that individual ; the continua- 

 tion of the kind being secured by the detachment of portions which tend 

 to run through the same cycle of forms as the parent. No forms of matter 

 which are either not living or have not been derived from living matter 

 exhibit these three properties, nor any approach to the remarkable phe- 

 nomena defined under the second and third heads." (Encyclopedia Bri- 

 tannica, 9th ed., art. " Biology," vol. iii. p. 679.) 



For the purposes of biological study life must be regarded as 

 a property of a certain kind of compounded matter. But we 

 are forced to regard the properties of compounds as the result- 

 ants of the properties of their constituent elements, even though 

 we cannot well imagine how such a relation exists ; and so in the 



