10 ZOOLOGY 



to observe what happens when one part is thrown out of gear, 

 but this method is subject to serious limitations. When one 

 part is diseased, as St. Paul has said, all the other parts suffer 

 with it, i.e. do not act normally, and it is sometimes difficult 

 to separate the effects due to the absence of the working of a 

 particular part from the effects due to the abnormal working 

 of the other parts. But in the various groups of animals 

 which are most nearly allied to man we have bodies made 

 up of the same parts as our own bodies ; but these parts are 

 of different relative sizes, and we find that when one part is 

 greatly enlarged a particular kind of behaviour is exaggerated, 

 and that when it is relatively much diminished certain ele- 

 ments of behaviour tend to disappear ; and we can reason with 

 complete confidence from the exaggerated structure to the corre- 

 sponding behaviour, and yet we are dealing with organisms in 

 which all the parts are working harmoniously together. To 

 give a very simple instance : there is a band of fibres connect- 

 ing our two eyes beneath the brain which is called the optic 

 chiasma. It is believed that the function of this is to focus 

 the two eyes on a single object so that it can be seen distinctly. 

 This belief is borne out by two facts : first, in ordinary fish 

 which have more or less blade-like bodies the optic chiasma is 

 absent : this is what is to be expected if our view as to the 

 function of the chiasma is correct, for such animals could by 

 no possibility focus the eyes which are on opposite sides of 

 their bodies on the same object ; secondly, in insectivorous birds 

 the optic chiasma is extremely large, and this again is what is 

 to be expected, for such birds which pursue their minute prey 

 on the wing have need of extremely accurate focussing, and 

 owing to the speed at which they travel must rapidly alter their 

 focus as they approach their prey. For the education of any 

 man who wishes to deal in an intelligent and progressive 

 manner, and not merely in a traditional manner with the 

 ailments of the human body a preliminary training in 

 zoology would appear to be absolutely necessary. 



But the study of zoology is of enormous importance on 

 many other grounds. A large part of our food is furnished 

 by the bodies of animals. Proteids, which form a necessary 

 part of our sustenance, can be obtained most directly from 

 this source. A few semi-civilised races are exclusively plant- 



