42 Chemical Basis of Genus and Species 



common chemical processes. Cohn, on the contrary, 

 considered the different forms among these red bacteria 

 (they are today called sulphur bacteria since they 

 oxidize the hydrogen sulphide produced by bacteria 

 of putrefaction to sulphur and sulphates) as definite 

 and distinct species, in spite of their common colour 

 and their association. Later observations showed that 

 Cohn was right. Winogradsky ' succeeded in proving 

 by pure culture experiments that each of these different 

 forms of sulphur bacteria was specific and did not give 

 rise to any of the other forms of the same colour found 

 in the same conditions. 



The method of pure line breeding inaugurated by 

 Johannsen'' has shown that the degree of definiteness 

 goes so far that apparently identical forms with only 

 slight differences in size may breed true to this size; 

 but for reasons which will become clear later on we 

 may doubt whether they are to be considered as definite 

 species. 



The fact of specificity is supported by the fact of 

 constancy of forms, de Vries has pointed out that 

 regardless of the possible origin of new species by muta- 

 tion the old species may persevere. Walcott has found 

 fossils of annelids, snails, crustaceans, and algae in a 

 precambrian formation in British Columbia whose age 



^ Winogradsky, S., Beitrdge zur Morphologic und Physiologie der 

 Bacterien. Leipzig, 1888. 



' Johannsen, W., Elemente der exacten Erblichkeitslehre. 2d ed., 1913. 



