CHAPTER XLVII 



INTRODUCTION TO COMPARATIVE ANATOMY 



In the study of Comparative Anatomy a method somewhat different 

 from our study up to this moment must be brought into play. 



In the forepart of this book the frog was studied as a type-form of 

 vertebrates, and the earth-worm as a type-form of annelids, as well as 

 of coelomates, and then, after each such type-form had been studied, it 

 was compared with other forms likewise studied in the laboratory. 



Now we are to take an entire system in each of the leading groups of 

 vertebrates and compare system by system, always reviewing the devel- 

 opment of the particular system studied, and comparing such develop- 

 ment with the development of the respective systems in both frog and 

 chick, as shown in part three (embryology) of this book. 



Three distinct points of view must be kept in mind in comparative 

 anatomy : 



Structure (both gross and microscopic). 



Development (embryology). 



Comparison of organ systems. 



Just as in any account of man's history we attempt to study those 

 races which we now consider as living under primitive conditions, be- 

 lieving that they will throw some light upon the problems that our an- 

 cestors had to overcome in order to bring about our present state of 

 civilization, so, in comparative anatomy we attempt to study the so-called 

 lower-animal life in order that this may throw light upon the develop- 

 ment of our own bodies. This may be brought home the better by re- 

 membering that all higher forms of life practically possess all organs and 

 systems of organs that the lower forms possess, plus an additional some- 

 thing. This does not prove by any means that any of the higher systems 

 must have necessarily come from the lower. All it does mean is that 

 all forms of animals which walk on the earth must have much that is 

 similar. For instance, legs that are used for the same purpose in all 

 animals must have muscles that will function alike ; because, regardless 

 of what position we systematically assign these animals, they, by virtue 

 of the fact that they must walk, must necessarily have leg muscles, and 

 having these, there must be a supporting structure for such muscles, 

 so that the skeletal systems of walking animals will be closely akin. 



Comparative Anatomy proper, then, will consist of a comparison of 

 the organ systems of four great groups of vertebrates. The classic ex- 

 amples used for such comparison are : 



The dogfish as an example of a group of living organisms whose 

 skeletal tissues are largely cartilaginous. 



The turtle as an example of the reptilia. 



