No. 496] ASPECTS OF THE SPECIES QUESTION 245 



with genera and life history as in the ease of other 

 plants? In the first place, morphological characters are 

 too ill-defined, if indeed they exist, and so he was forced 

 to devise some other means of discrimination. This 

 means was at first almost purely physiological. It 

 largely depended upon the behavior of the individual 

 sorts, or the groups of individual sorts, to certain nutri- 

 tive processes. It depended upon what kind of media 

 the bacteria grow upon, the temperature, and access to 

 oxygen. Upon these physiological results in large part 

 have been built our vast knowledge, economic very 

 largely, of the many kinds of bacteria which are desig- 

 nated as species. But we may pass by the bacteria for a 

 time and take up the larger organisms. I think the ones 

 that are most useful in this connection are the strictly 

 parasitic forms. 



There is no more interesting and better understood 

 group at the present time than the plant-rusts. The 

 forms are sufficiently large to furnish a variety of species 

 and genera exhibiting morphological characters, and yet, 

 being strictly parasitic, are particularly restricted by the 

 nature of the substratum. I will give one or two in- 

 stances to illustrate. There is a group of forms— I will 

 admit that I hardly know what terms to use in speaking 

 of them, but I will say a group of forms-which grow 

 upon the various species of Oarex. It is found that by 

 taking rust spores from a single host of any particular 

 Carex and sowing them upon an Aster, or a Solidago or 

 an Erigeron, they will grow upon one of these genera, 

 it makes not much difference what the species, but not 

 upon the other two. Now if spores are taken from 

 another Carex, the spores being so exactly like the former 

 that they can not be distinguished by any visible char- 

 acters, and sown upon plants of the same three genera, 

 they may grow upon a different one than in the former 



we will get three sets of forms, one growing on Aster, 

 one on Solidago and one on Erigeron, which can not be 



