364 A HISTORY OF FISHES 



(Fig. 132a). The branches marked a and b arc totally extinct, 

 while that marked c is moderately successful and represented 

 at the present day by a few species. The remaining iDranches, 

 D and E, ha\e been most successful, and the topmost twigs 

 show an abundance of closely allied living species. The second 

 diagram (Fig. 132b) represents a part of one of these end 

 branches in greater detail, and illustrates the manner in which 

 species may become separated by the dying out of connecting 

 links. Here the dark lines represent the separate individuals, 

 and the enclosed areas are the limits of existing species. All 

 the individuals within each of these areas, although differing 

 from one another in minor characters, are fundamentally alike, 



-'\ C 



B 







- a.. ii b. 



Fig. 132. 



DIAGRAMS OF PORTIONS OF TYPICAL PHYLOGENETIC " TREES." 



(For explanation see text.) 



and the three groups of individuals shown in the diagram 

 have become well differentiated by the dying away of the 

 branches from which they sprang. Further, the two on the 

 right, marked a and b, may be seen to diverge from a common 

 stem at a point not very far down the tree, whereas that marked 

 c joins the same branch at a point nearer to the base. The 

 species a and b, therefore, might reasonably be expected to 

 resemble each other more closely than either resembles c, and 

 they are consequently to be regarded as representing one genus, 

 while G forms another. 



The lowest unit in any classification is, of course, the indi- 

 vidual, and these must first of all be grouped into larger units 

 or species. The Latin word species means literally a particular 



