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INTRODUCTORY: THE ANIMAL ORGANISM 



BIOLOGY 



Zoology, the science of animals, is a branch of biology, the 

 science of living beings. Of such beings there are various kinds 

 besides the animals ; but all the kinds have important features 

 in common. Rightly to understand any animal is therefore to 

 comprehend properties which it shares with non-animal living 

 beings as well as its purely animal characters. Thus at the outset 

 of our study of zoology it is desirable that we should spend a 

 little time in considering the nature in general of living beings. 



LIVING AND LIFELESS 



Out of the multitude of material objects that surround us, 

 we distinguish some, as alive, from the rest, which are lifeless 

 — that is, either are dead or have never been alive. It would be 

 helpful if we could set down in precise terms the properties on 

 which we base this distinction. 



The commonsense view is that a living being, such as a cow 

 or a tree, grows and reproduces, while a non-living thing, such as 

 a stone or a piece of wood, does not, but a little reflection will 

 show that no precise distinction can be made in these terras. A 

 cow may be capable of reproduction, but for a variety of reasons, 

 such as absence of a bull or infection by a parasite, may not 

 in fact produce offspring ; an ox can never reproduce, but is 

 nevertheless alive, and if it be objected that an ox is in an artificial 

 state through the interference of man, there is the parallel 

 example of the worker bee, which is sterile by nature. It is a 

 matter of common observation that growth, in the ordinary sense 

 of the word, ceases in man relatively early in hfe, and in old age 

 it becomes negative, if the expression may be permitted. In 

 some of the lower animals, too, ' degrowth ' is known ; thus, if 

 a pond fiatworm is starved it decreases in size and appears to 

 reverse its normal trend. 



No better definition of life can be made from any of the other 

 properties which have from time to time been proposed for the 



