ii8 The Inside s and the Wordings of Living Things 



scope. The attempt to grow these in different kinds of fluids 

 at once reveals important fundamental differences. And 

 these differences are consistent from generation to generation. 

 The microscope is unable to distinguish for us various kinds 

 of *' typhoid " bacilli or " pneumonia " germs. But there 

 are different kinds, as we can discover by growing them in 

 tubes and by finding how they modify the life processes in 

 an experimental animal. 



Among higher plants also we find different strains that 

 cannot be distinguished by their appearance, but can be 

 separated by their chemical qualities. When the manufac- 

 ture of black dye from logwood was still an important in- 

 dustry it was necessary to discover a way of distinguishing 

 good logwood trees from those that turned out after being 

 cut and ground up to be utterly worthless for dye pro- 

 duction. Up to the time when organic chemistry supplanted 

 the logwood dyes, no such method had been found. In rais- 

 ing sugar beets or plants for the production of drugs, the 

 same problem appears. It has been possible to segregate and 

 breed varieties with a high proportion of the desired sub- 

 stances; but this was brought about not by finding differences 

 in the appearances of different strains, but by making chemi- 

 cal analyses of samples and segregating for breeding purposes 

 those that contained the desired material in large proportions. 



Convergence of Evidence 



To be sure, the subtle similarities which are revealed 

 by the sensitized serum of the laboratory animal may be 

 merely a coincidence. The significance of the coincidence 

 lies in its consistent appearance in thousands of cases selected 

 from wide ranges of plants and of animals, and in the paral- 

 lelism between these coincidences and the coincidences re- 

 vealed by fossils in the rocks and by meaningless structures 

 found in the bodies of animals, and of plants. The more 

 intimately plants and animals are studied, whether in their 

 structure or in their ways of meeting the demands of life, 



