Classification. 81 



two allied forms, so in the new there is the same difficulty with the biological 

 characters, but it may be laid down, as a general rule, that the sharper the 

 distinction between two biologically different forms, the greater the reason 

 for calling them species, while the less distinctly marked would be called 

 biologic forms. lit is all a matter of degree, and it must be left to the 

 good tact of the investigator, as Klebahn says, where to draw the line 

 between species and biologic forms. 



Among the heteroecious rusts, there may be structural resemblances 

 between one generation and differences in the other. In that case, as a 

 matter of convenience, if the differences in the one generation are sufficiently 

 distinct, such should be designated species. 



Biologic forms, in fact, may be regarded as incipient morphological 

 species, the physiological differences at present existing becoming ultimately 

 associated with morphological distinctions, which will mark them off as true 

 species. It is not easy to explain how this has come about, but we 

 may be sure that the explanation is to be sought, not along one line, but 

 along various converging lines. 



The biologic forms of one and the same morphological species, such as 

 Puccinia graminis, may be supposed to have had a common origin, and the 

 most probable view is that the original form inhabited all the hosts on 

 which its descendants now live, and some of these descendants specialised 

 on one or other of these common host-plants. What caused them thus to 

 specialise? It may have been either through gradually becoming adapted 

 to certain of these host-plants in preference to others, or due to a spontaneous 

 change arising from internal causes, as expressed in the mutation theory 

 of De Vries. 



This is Fischer's 4 view, and Klebahn 1 , after examining the evidence, 

 sums up as follows: "The manifold characters of the existing biological 

 species and races appear to have come about owing to the alternating exten- 

 sions and restrictions of the area of nutritive plants.. These changes, and 

 especially the restrictions of area, have been influenced by adaptation and 

 selection, but many observations indicate that internal developmental tenden- 

 cies have also played a part in determining the direction of the evolution." 



The whole trend of this modern investigation is towards the recognition 

 of more deep-seated characters in the discrimination of species. One para- 

 site has become so thoroughly adapted to the physiological characters of a 

 host-plant that it cannot infect another, and so the plant on which the para- 

 site lives becomes a diagnostic feature of it. It is so much easier, and has 

 been so long the custom to be guided by morphological characters alone, 

 that physiological distinctions are not readily accepted, but now that they 

 are known, they must be recognised in some way, and the most convenient 

 is to incorporate them in the specific characters. 



Again, the different stages of a fungus require to be known, in order 

 to classify it properly, and the mere knowledge of the uredo or aecidial 

 stage will not suffice for this purpose. Hence the life-history and infective 

 power, as well as structural characters, must all be taken into account in 

 fixing and determining species. 



