BACTERIAL CLASSIFICATION 25 



but in space of two dimensions, as it were, so that the 

 common types stand out as mountain tops above their 

 fellows, each mountain connected by valleys of inter- 

 mediate types with many of its neighbors. If now the 

 mountains were cut off by a horizontal plane half way 

 up their sides and attention were paid only to the moun- 

 tain tops, disregarding the valleys, we should have the 

 popular conception of species. The biologist, on the 

 contrary, is more concerned with the intermediate types 

 in the valleys, as illustrating variation and the connection 

 between allied species. In some groups of plants and 

 animals the mountains are few and high and the valleys 

 very deep. These are the groups which are, so to speak, 

 in a stationary condition — which are not rapidly 

 varying and adapting themselves to new conditions. In 

 other groups, which biologists call 'dominant genera,' 

 the mountain tops are numerous but not so high and 

 separated by only shallow valleys; these are the groups 

 which are at the moment succeeding in the struggle for 

 existence." (Andrewes and Horder, 1906.) 



By the application of these methods, Andrewes and 

 Horder succeeded in bringing a reasonable order out of 

 the chaos which had shrouded the systematic relation- 

 ships of the streptococci. Years of previous work by 

 scores of able bacteriologists had only made it clear 

 that this genus included an innumerable series of vari- 

 eties; not a single step had been taken towards the 

 arrangement of these varieties in natural groups. The 



