14 ZOOLOGY 



rate, and all sorts of problems have sprung up in consequence. 

 The nature of these problems is clear, and their occurrence 

 seems inevitable to the man who has had a zoological training, 

 but it is far from clear to many of our so-called thinkers who 

 have not had this advantage, otherwise we should not hear 

 proposals put forward by Fabian societies and similar organi- 

 sations that the State should undertake the care and nurture of 

 the babies of the poor, thus relieving the least competent 

 members of the State of the last deterrent to reckless repro- 

 duction. If we are going to try to prevent Nature's method 

 of choosing the best, viz. the elimination of the poorest and 

 weakest, we must surely substitute for it our own selection, 

 else we shall suffer the fate which befalls half-hearted meddlers 

 in great affairs. 



CHAPTER III 



THE LIVING SUBSTANCE. 



HAVING gained some conception of the nature and bearing of 

 zoological science, we may now proceed to examine some of the 

 special problems with which it deals. In a book of such 

 limited size as the present volume it would be futile to attempt 

 to cover the whole extent of the science : the most that can be 

 done is to try to select a few samples of the kind of question 

 that zoologists have tried to solve. 



All life expresses itself in movement, either, as in the case 

 of plants, in the movement due to growth or, as in the case of 

 animals, in the r^ovement of fully grown parts, which return 

 after their movement to their former condition Now if we 

 leave aside for the present the movement due to growth and 

 confine our attention solely to the characteristic animal move- 

 ment of adult parts, the first question which confronts us is 

 this : How is this movement produced? If we examine small 

 fragments or very thin slices of the part of the body which is 

 the seat of movement, we shall always find clear fibres, which 

 when movement occurs grow shorter and thicker. These fibres, 

 which are termed muscle fibres, are the direct cause of move- 

 ment in all the higher animals. In the lower animals we 



