Xviii INTRODUCTION. [CH. 



juster views would prevail as to the distinction of spe- 

 cies^ and the well- deserved reproach of those philo- 

 sophers who are confirmed in their denial that species 

 have any real existence in nature by the notorious dis- 

 agreement of naturalists as to their limits would thus 

 cease. There is no judge or ^' arbiter specierum ; ^^ and 

 every naturalist is at perfect liberty to follow the bent 

 of his own discretion or inclination in the extension or 

 reduction of species^ subject only to the opinion of his 

 scientific compeers. He is amenable to no other autho- 

 rity. Hence arises that great and continual diversity of 

 opinion as to the determination of certain species among 

 natui'alists, whose opportunities or experience are more 

 or less great J or whose minds are differently constituted — 

 the nature of some being rather analytical and of others 

 synthetical. The same remark of course applies to the 

 distribution of species into genera^ and of these, again, 

 into families and higher groups or sections. To a cer- 

 tain extent all classification is artificial and arbitrary; 

 but the necessity for some arrangement of the kind is 

 obvious, considering the immense number and variety 

 of objects to which some name or symbol of distinction 

 must be given for the sake of those who pursue or study 

 any branch of natural history. Systems of classification 

 are as indispensable to a naturalist as tools to a work- 

 man. This necessity of science equally applies to the 

 discussion of the interesting question as to the origin 

 and mutability of species, which in the absence of such 

 data could not be satisfactorily conducted. 



Varieties. — Besides species, and holding a subordinate 

 rank to them in the great host of Nature's works, are 

 certain forms called " varieties,^' which are not less defi- 

 nite, but more difiicult to separate from the typical or 

 specific forms. They are offshoots of species, and origi- 



