LIVING MATTER. 5 



waste resulting from the decomposition of the molecules of the protoplasm 

 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 car- 

 bon, 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 stationary, 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 intus-susception, and therefore differs 

 altogether from the process of growth by accretion, which may be ob- 

 served in crystals, and is effected purely by the external addition of new 

 matter; so that, in the well-known aphorism of Linna?us, 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 ex- 

 istence. The new form takes on the characters of that from which it 

 arose; exhibits the same power of propagating itself by means of an off- 

 shoot; and, sooner or later, like its predecessor, ceases to live, and is re- 

 solved into more highly oxidated compounds of its elements. 



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

 stance, but its size and form are undergoing continual modifications, the 

 end of which is the death and decay of that individual; the continuation 

 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." (ISncyclopcedia Bri- 

 tannica, 9th ed., art. "Biology," vol. iii., p. 6T9.) 



Concerning the chemical composition of living matter and its 

 significance, a few considerations should be borne in mind. For 

 the purposes of biological study life must be regarded as a prop- 

 erty of a certain kind of compounded matter.* But we are 



* Herbert Spencer, Principles of Biology, vol. 1. 



