2 COMPARATIVE PHYSIOLOGY. 



But let us first inquire : What are the determining charac- 

 teristics of living things as such ? By what barriers are the 

 animate and inanimate worlds separated ? To decide this, falls 

 within the province of general biology. 



Living things grow by interstitial additions of particles of 

 matter derived from without and transformed into their own 

 substance, while inanimate bodies increase in size by superficial 

 additions of matter over which they have no power of decompo- 

 sition and recomposition so as to make them like themselves. 

 Among lifeless objects, crystals approach nearest to living 

 forms ; but the crystal builds itself up only from material in 

 solution of the same chemical composition as itself. 



The chemical constitution of living objects is peculiar. Car- 

 bon, hydrogen, oxygen, and nitrogen are combined into a very 

 complex whole or molecule, as protein ; and, when in combina- 

 tion with a large proportion of water, constitute the basis of all 

 life, animal and vegetable, known as protoplasm. Only living 

 things can manufacture this substance, or even protein. 



Again, in the very nature of the case, protoplasm is continu- 

 ally wasting by a process of oxidation, and being buUt up from 

 simpler chemical forms. Carbon dioxide is an invariable prod- 

 uct of this waste and oxidation, while the rest of the carbon, 

 the hydrogen, oxygen, and nitrogen are given back to the in- 

 organic kingdom in simpler forms of combination than those 

 in which they exist in living beings. It will thus be evident 

 that, while the fiame of life continues to burn, there is constant 

 chemical and physical change. Matter is being continuously 

 taken from the world of things that are without life, trans- 

 formed into living beings, and then after a brief existence in 

 that form returned to the source from which it was originally 

 derived. It is true, all animals require their food in organized 

 form — that is, they either feed on animal or plant forms ; but 

 the latter derive their nourishment from the soil and the atmos- 

 phere, so that the above statement is a scientific truth. 



Another highly characteristic property of all living things 

 is to be sought in their periodic changes and very limited dura- 

 tion. Every animal and plant, no matter what its rank in the 

 scale of existence, begins in a simple form, passes through a 

 series of changes of varying degrees of complexity, and finally 

 declines and dies ; which simply means that it rejoins the in- 

 animate kingdom : it passes into another world to which it 

 formerly belonged. 



