IMPORTANCE OF COMPARATIVE ANATOMY. 245 



limbs, etc. from the simple Gastrula. The two primary 

 germ-layers which form the entire body of the Gastrula fall 

 by fission into the four secondary germ-layers, which have 

 already been named ; and of these four the whole complex 

 form of the perfected human and animal body constructs 

 itself. It is so difficult to understand this process of con-' 

 struction, that we will first look around us for an ally 

 capable of helping us over many obstacles. 



This powerful ally is the science of Comparative 

 Anatomy. Its object is, by comparison of the perfected 

 bodily forms of the various groups of animals, to discover 

 the universal structural laws, in accordance with which the 

 animal body develops ; and at the same time, by critically 

 determining the degrees of difference between the various 

 classes, and the larger groups of animals, to establish their 

 relations to each other and to the whole system. There was 

 a time when this task was attempted from a teleological 

 point of view, and in the actually existing apt organization 

 of animals proof was sought of a pre-arranged "plan of con- 

 struction" by the Creator; but, recently, the establishment 

 of the Theory of Descent has enabled Comparative 

 Anatomy to go deeper, and its philosophical task has de- 

 veloped into the explanation of the variety of organic forms 

 by Adaptation, and their similarity by Heredity; it has 

 also to seek to discover the various degrees of blood- 

 relationship in the graduated and various form-relationships, 

 and to prove as nearly as possible the genealogy of the 

 animal kingdom. In this way Comparative Anatomy is 

 most closely allied to the classification of organic bodies, 

 which, starting from the opposite direction, aims at thu 

 same result 



