84 THE INVISIBLE WORLD 



This process of selective culture on divergent 

 lines from the same ancestor is paralleled in thou- 

 sands of instances in the higher forms of vegetable 

 and animal life. Take a single example: 



The modern dray horse and the trotter have both 

 descended from the same wild ancestor. By selec- 

 tive breeding, and culture along one line, has come 

 the heavy weight horse for hauling heavy loads. 

 By selective breeding, and culture along a differ- 

 ent line, has come the light weight horse for speed. 

 It takes about two and a half trotters to make in 

 weight one dray. Each, in a poor way, may do the 

 other's work; but each can do the work, for which 

 it has so long been trained, a thousand times bet- 

 ter. 



The parallel is most complete. The microbe of 

 beer and the microbe of bread, coming from the 

 same wild ancestor, now differ one from the other, 

 in their tame and domesticated state, quite as much 

 as the dray differs from the trotter. The beer mi- 

 crobe is almost exactly two and a half times as 

 large as the bread microbe. The beer microbe can 

 also do the work of the bread microbe : that is, the 

 baker can bake Kis bread with the brewer's yeast. 

 And the microbe of bread can do the work of the 

 microbe of beer; that is, the brewer can brew his 

 beer by using the baker's yeast. But the result in 

 either case is quite as unsatisfactory as it is when 

 the dray and the trotter exchange their work. 

 Each microbe does its best only when used on that 

 line of work for which it has so long been trained. 



