SEC. n PRINCIPLES AND PRACTICE OF CLASSIFICATION 117 



Turdus " in the abstract is simply that form ; and that form is 

 prototypic of its derivatives. In the concrete, as represented 

 by its teleotypes, the genus Turdus sums the modifications which 

 these have collectively undergone, without specifying the particular 

 modifications of any of them ; it expresses the way in which they 

 are all like one another, and in which they are all unlike the re- 

 presentatives of any other genus. Thus what is above advanced 

 is seen to hold, though genera and all other groups are actual 

 descendants of individuals specifically identical. 



Generalised and Specialised Forms. — Taking any one group 

 of animals — say the genus Turdus, of numerous species — and con- 

 sidering it apart from any other group, we perceive that it represents 

 a certain assemblage of characters peculiar to itself, aside from those 

 more fundamental ones it includes of its family, order, etc. Its 

 particular characters we call "generic." Among the numerous 

 teleotypic forms it includes, there is a wide range of specific varia- 

 tion, within the limits of generic relationship. Some of its species 

 are modified farther away than some others are from the generic 

 standard or type to which all conform more or less perfectly. The 

 former, having more peculiarities of their own, are said to be the 

 most specialised ; the latter, having fewer peculiarities, are the least 

 specialised. Those that are the least specialised are obviously the 

 most generalised ; and this means that we believe them to be nearest 

 to the stock whence all have together descended with modification. 

 The application of this illustration to great groups shows us the 

 principle upon which any form is said to be generalised or specialised. 

 The Ichthyornis, with its fish-like vertebrae, reptile-like teeth, bird- 

 like sternum and shoulder-girdle, is a very generalised form. A 

 thrush is the opposite extreme of a highly specialised form. The 

 two are also separated by an enormous interval of time ; one being 

 very old, the other quite new ; a chronological sequence is here 

 perceived. Since the evolutionary processes concerned in the modi- 

 fication on the whole represent progress from simplicity to com- 

 plexity of organisation, and therefore ascent in the scale of 

 organisation, a generalised type, an ancient type, and a simple type 

 are on the whole synonymous, and to be contrasted with forms 

 specialised, recent, and complex. They therefore respectively corre- 

 spond to 



" Low " and " High " in the Scale of Organisation. — All 

 existing birds are very closely related, notwithstanding the great 

 numerical preponderance of the class in the present geological 

 epoch. This outbreak, as it were, of birds upon the modern scene, 

 is like the nearly simultaneous bursting into bloom of a mass of 

 flowers at the end of one branch of the Sauropsidan stem. All 

 modern birds, in fact, are strongly specialised forms, so much so that 



