2 INTRODUCTION. 



and not merely fanciful, bond of close affinity. The search 

 for the natural genealogy of these families of organisms is one 

 of the grandest of the problems propounded to modern zoology 

 by the great English philosopher. 



Naturally enough, in this search, zoologists had recourse to 

 those means and methods which were most familiar to them and 

 which had hitherto been at their disposal. Ever since the time 

 of Cuvier they had been accustomed , to discriminate between 

 the different forms of animals and to describe the organs which 

 distinguish and separate them, endeavouring at the same time 

 to detect by them the ideaV affinities of animal types. But they 

 were more practised in the use of the scalpel and microscope 

 than in availing themselves of the often highly complicated 

 apparatus and methods of the physiologist, and it is only lately, 

 under the influence of Darwin's views, that they have begun 

 to enquire into the true and natural affinities of animal types 

 by comparing them together as to form, and by studying their 

 mode of origin. Thus it is that the modern study of animal 

 morphology has arisen, commonly divided into comparative 

 anatomy and embryology ; but at the same time, equally under 

 the influence of Darwin, zoologists began to devise genealogical 

 trees for the different groups of the animal kingdom — sometimes 

 for a whole group, sometimes for a subdivision only — ^in which 

 they attempted to give graphic expression to such knowledge as 

 they supposed they had acquired of the actual processes which, 

 through constant modifications of the most widely different 

 forpis, led finally to the development of the human body. 



Of course such pedigrees could not be otherwise than of a 

 somewhat doubtful character. In all zoological investiga- 

 tions, as in almost everything else, a certain influence may 

 be detected which may be termed the personal element. 

 Zoologists are not, as mathematicians are, able to set out from 

 certain immutable axioms, and to calculate from them the forms 

 and origination of animal types with mathematical exactitude ; 

 on the contrary, they are forced to deduce all the laws of their 

 science from observations of phenomena. The mode of carrying 

 Ion these observations, moreover, and consequently the answer 

 which nature gives to the questions put to her, depend 



