COMPARATIVE ANATOMY 



CHAPTER I 

 THE ANIMAL KINGDOM 



Since some of the so-called lower animals, li\dng or extinct, more or 

 less resemble hv-pothetical ancestors of man, some knowledge of them is 

 necessary for a proper understanding of the history of the human body. 

 Moreover, certain highly complex and obscure organs of man are most 

 easily understood in the light of the simpler conditions of lower forms. 

 Even the plants, so unUke us in outward appearance, contribute something 

 to our knowledge of ourselves. 



But the organic world is so enormously complex that no human mind 

 can carry its detail adequately without some system by which facts are 

 classified and summarized. Most useful of such systems are those based 

 on natural relations, which, therefore, exhibit the course of evolution of 

 each species, and place it correctly in an evolutionary scheme. For 

 evolution, nowadays, is the key to all genetic animal relationships. 



Such an evolutionary scheme begins by dividing all Hving things into 

 plants and animals. Plants are creatures which contain chlorophyl, and 

 therefore, can, produce or make their food directly out of inorganic mate- 

 rials, or else they are, ob\dously, such creatures as have lost their chloro- 

 phyl and adopted the feeding habits of the simpler animals. Animals 

 may or may not have descended from plants; only rarely do they contain 

 chlorophyl, hence all their structure and habits rest on other means of 

 obtaining food. There are, however, many simple organisms; for example, 

 the slime molds, which are as much one as the other, plants or animals 

 indifferently. Even some of the higher plants, Uke the venus fly-trap, 

 catch and devour insects; and some of the unicellular algae also feed like 

 animals. 



The animal kingdom as a whole is commonly di^dded into about a 

 dozen phyla, the precise number and the precise definitions of which have 

 not yet been agreed upon by taxonomists. These phyla, in turn, are split 

 into classes, the classes into orders, the orders into genera, and the genera 

 into species. It is sometimes convenient, also to recognize sub-orders and 

 sub-classes, and to combine similar genera into families. 



Scientific naming is by genera and species, a scheme devised by the 

 great naturahst Linnaeus, or Linne, about the middle of the eighteenth 



