THE ANIMAL ORGANISM 



purpose. Neither irritability, nor self-preservation, nor respira- 

 tion is at all times characteristic of things which are called 

 living, and one or more of these properties (as well as growth and 

 reproduction) may at times be found in things which are called 

 non-living. In the limit, it remains a matter of opinion w^hether 

 the viruses (small particles which cause a number of diseases 

 such as smallpox and measles) should be considered as living or 



non-living. 



In spite of the impossibihty of making a definition, there are 

 certain properties which are generally found in things which men 

 tacitly agree to call living, and which are seldom found in things 

 which they do not so call. We w^ill now briefly consider the more 

 important of such properties. 



GROWTH 



Growth means the increase in quantity of material in a body, 

 and is usually measured, for biological purposes, as increase in 

 dry weight (this word being used incorrectly for mass), that is, 

 weight after free water has been driven off at 105° C. It thus 

 excludes mere addition of water, which may be so rapid and tem- 

 porary as to be meaningless, and differs fundamentally from 

 increase in size, which may occur by absorption of water or even 

 air. It depends on an uptake of matter from the surroundings, 

 and on this more is said below. 



REPRODUCTION 



An animal or plant may, by itself or in co-operation with 

 another, produce a new living thing, a process known as repro- 

 duction. In a sense reproduction is a consequence of growth, for 

 growth without reproduction would lead to an impossible expan- 

 sion in size ; this connection is seen at its clearest in an animal 

 Hke Amceha which simply divides into two when it reaches a 

 certain size, but in the higher animals the connection is obscured. 

 The division of a cell of a higher organism (p. 688) though it may 

 take place by a similar mechanism, is philosophically distinct 

 from the reproduction of Amceha. 



Reproduction always includes, though it may be much more 

 than, the fission or division of an existing body. Whatever may 

 have been its origin, all the evidence suggests that under the 



