6 INTRODUCTORY [CH. I. 



there are certain cells which still retain their primitive structure ; 

 notable among these are the white corpuscles of the blood. 



FIG. 4. Human colourless blood-corpuscle, showing its successive changes of outline within 

 ten minutes when kept moist on a warm stage. (Schofield.) 



It would appear at first sight an easy problem to distinguish 

 between a living thing, and one which is not living. The principal 

 signs of life are the following : 



1. Irritability ; that is the property of responding by some change 

 under the influence of an external agent or stimulus. The most obvious 

 of these changes is movement (amoeboid movement, ciliary movement, 

 muscular movement, etc.). 



2. Power of assimilation, that is, ability to convert into protoplasm 

 the nutrient material or food which is ingested. 



3. Power of growth ; this is a natural consequence of the power 

 of assimilation. 



4. Power of reproduction ; this is a variety of growth. 



5. Power to excrete ; to give out waste materials, the products of 

 other activities. 



It should, however, be recognised that certain of these five char- 

 acteristics may be absent or latent, and yet the object may be living. 

 For instance, power of movement is absent in many vegetable struc- 

 tures ; certain seeds and spores can be dried and kept for many years 

 in an apparently dead condition, and yet will sprout and grow when 

 placed in appropriate surroundings. 



Of all the signs of life, those numbered 2 and 5 in the foregoing 

 list are the most essential. Living material is in a continual state 

 of unstable chemical equilibrium, building itself up on the one hand, 

 breaking down on the other ; the term used for the sum total of these 

 intra-molecular rearrangements is metabolism. The chemical sub- 

 stances in the protoplasm which are the most important from this 

 point of view are the complex nitrogenous compounds called Proteins. 

 So far as is at present known, protein material is never absent from 

 living substance, and is never present in any thing else but that 

 which is alive or has been formed by the agency of living cells. It 

 may therefore be stated that Protein Metabolism is the most essential 

 characteristic of vitality. 



