INTRODUCTION 13 



and the limbs of the lobster perform the same kinds of func- 

 tions as in the fish, they may be said to resemble one another. 

 But the resemblance is only one of use and function, not of 

 form, and organs which perform similar functions whilst differ- 

 ing in plan in different animals are said to be analogous. 



On the other hand, the eyes, skull, backbone, and numerous 

 other parts of the dog and fish clearly resemble one another 

 in plan of structure. Such organs are said to be homologous. 



It is most important that the student should learn to distin- 

 guish between organs which are merely analogous and those 

 which are homologous. Comparative anatomy is largely con- 

 cerned in establishing homologies between organs which often 

 differ considerably from one another in superficial aspect and in 

 function. In these days we consider organs to be homologous 

 when we can produce evidence to show that they are modifica- 

 tions of some pre-existing organ belonging to a remote ancestor 

 from which all the animals possessing the organs in question 

 have descended. The nature of this evidence, and the reason- 

 ing which enables us to assert that the identity in the 

 architectural plan of certain animals is due to their having 

 inherited their leading features from a common ancestor, 

 whilst varying the details in an almost infinite degree, will 

 become apparent in the course of this work. 



We must now pass to the more detailed consideration of 

 a single animal type, and learn from it what is the structure 

 of the organs about which we have been talking, and what 

 relative positions they hold to one another. As we are more 

 familiar with the general structure of our own bodies than that 

 of other animals we should do best if we took the human 

 subject as a type; but the study of human anatomy is in 

 many ways inconvenient, and we shall take as our type the 

 common frog, an animal easily obtained, of a size convenient 

 for dissection and sufficiently like the human subject to 

 enable us to learn all the lessons necessary for the elementary 

 comprehension of our subject. 



Let it be understood here that the student will make no 

 progress in the study of comparative anatomy unless books and 

 lectures are supplemented by thorough painstaking practical 

 work. No words that ever were spoken or written, no draw- 

 ings or diagrams, however artistic or simple, can ever bring 

 home to the mind the structure of the animal body without 



