2 COMPARATIVE ANATOMY 



succeeds another, every generation differing somewhat, both 

 collectively and in the individual characteristics of its com- 

 ponent members. In fact, the successive generations of 

 mankind are but a part of the successive generations of living 

 beings ; and, whilst we use them as an illustration, we are only 

 specifying a part of that universal evolution which it is our 

 business to trace more closely. 



Whilst these principles of uniformity and evolution must be 

 our guides through the very intricate paths of the study of 

 animal organisation, it must not be supposed that they are 

 obvious and elementary truths, which can be arrived at by 

 a priori reasoning. On the contrary, they have been attained 

 to only as the result of an immense amount of study and reflec- 

 tion on the details of animal organisation, and they cannot be 

 apprehended without a very considerable amount of study and 

 reflection on the part of every individual. It is the object of 

 the present course to justify our belief in these principles by 

 the study of just so much of the structure and development of 

 certain animals as will suffice to bring conviction to our minds. 



Superficially, the animal world seems to be in a condition 

 of comparative stability. The least observant person already 

 recognises in early childhood a considerable number of kinds 

 of animals. The individuals of each kind are approximately 

 like to one another, they come and go, are born and die, but 

 they leave behind them progeny which continues to exhibit 

 the same characteristics of form and structure, and we learn 

 from books that these same kinds of animals with which we 

 are familiar, have existed with these same characteristics ever 

 since man began to describe the natural objects by which he 

 is surrounded. 



Daily experience and written record, therefore, combine to 

 impress upon our minds the notion of the fixity of the form 

 and structure of the different kinds of animals. The fixity is 

 not absolute, for we recognise individual differences between 

 animals of the same kind, yet these differences are so small 

 and the likenesses so great that we have no hesitation in 

 describing animals having such and such characters as being 

 of one kind, or, as zoologists say, of one species. 



We observe, then, that the earth is tenanted by countless 

 species of animals, that each species remains constant and has 

 remained constant as far back as human records go, yet we 



