66 COMPARATIVE ANATOMY. 



general nature, affecting the position or arrangement of the most 

 important systems of organs, and partly they affect the special 

 development of individual organs; they extend to agreement in 

 form, size, and number. The classifying spirit of man has formed 

 definite conceptions of these relations of organisms to one another. 

 All those individuals which agree in essential points he has called 

 a species, and has united into a genus those species which resemble 

 one another in a number of points ; these again he has united into 

 larger divisions, families, orders, and classes. Thus arose the 

 zoological system, which, in so far as it unites what agree, and 

 separates what differ, has come to be the expression of our general 

 knowledge of the Animal Kingdom. 



In this way the whole Animal Kingdom can be broken up into 

 several large divisions, each of which differs from the rest by a 

 number of special characteristics. The essential character may be 

 recognised in all the subdivisions, and even under great individual 

 variations. This has been called the "type." The type then 

 means a collection of characteristics which are expressed in the 

 organism, and which are predominant in a large division of the 

 Animal Kingdom, and which are evident in the course of develop- 

 ment as well as in the adult condition. Such larger divisions which 

 differ from others in certain fundamental points of organisation are 

 themselves called " types." 



Within each type we note a variation in the characters of the 

 divisions which make it up, and this often to such an extent that 

 what is characteristic of the type appears to be lost in some forms. 

 In this case it is always individual development which enables us 

 to recognise the connection of these forms with the " type." 



If we admit that similarity of organisation in different individuals 

 is explicable by the fact that they have a common ancestor, and that 

 therefore these similarities are due to affinity, we must regard less 

 close similarities as due to a less close relationship. We therefore 

 regard the individuals which belong to one species as more closely 

 allied than are the representatives of different species, and within 

 the limits of one species we shall again derive from common ancestors 

 those individuals which are distinguished by special characters, and 

 which we unite into a sub-species. 



No one has any hesitation in recognising within the limits of 

 small groups of individuals the phenomenon of the continuation of the 

 peculiarities of a given organism into other individuals by means of 

 transmission ; indeed it is often possible to perceive, by direct ob- 

 servation, that descendants are like their ancestors. By extending 

 this conception of affinity to a wider circle, and regarding what is 

 common in organisation as due to a common descent, and what is 

 divergent as due to adaptations, we take our stand on the theory 

 of Descent (cf. §§ 4 and 5). We consequently regard the large 

 divisions known as "types," as phyla, or leading branches of the 

 genealogical tree, and by so doing point to the cause which has 

 determined their existence. 



