BRIDGING SPECIES 71 



case out of many experiments could B. madritensis be infected 

 directly from B. mollis. B. sterilis therefore acted as a 

 "bridging" species, and enabled the parasite to pass from 

 B. mollis to B. madritensis, though it could not do so without 

 this intervention. The same existence of "bridging" species 

 has been demonstrated in Erysiphe graminis, and no doubt 

 will be found in numerous other instances. 



Ward further mentions (1903) that he found B. arduennensis 

 var. villosus to be infectible by the spores from B. sterilis, B. 

 mollis, and B. patulus, as well as by those from B. arduennensis, 

 and therefore easily able to serve as a bridging species between 

 these others. Nevertheless, that such intermediary species 

 do not exist in all cases is proved by the fact that when he 

 grew " more than 200 species and varieties of Bromus side by 

 side or intermingled in contiguous beds, certain species invari- 

 ably caught the disease and became rusted, while others close 

 by showed no sign of infection." 



The occurrence of these abnormal spores, i.e. mutations, 

 which is proved in the case of Bromus, is of great significance 

 and gives us the clue by which we can understand how a 

 gradual or sudden passage can take place, and has taken place, 

 from one host to another, so that now an appreciable percentage 

 of the modern vegetable world have parasites more or less 

 specialised to themselves. 



Another important consequence follows from this fact of 

 specialisation. If the parasite is so narrowly adapted to its 

 particular host, it may be expected that varieties of the host 

 can be found or bred which will be able to resist attack, that 

 is, will be immune. A great deal of research has been devoted 

 of late years, especially by Professor Biffen of the Agricultural 

 School at Cambridge, to this subject of breeding a race of 

 wheat which will be immune to Rust, and a certain amount of 

 progress has been made. Immunity depends chiefly (perhaps 

 entirely) upon the ability of the cytoplasm of the host-cells 

 to resist infection by secreting anti-toxins which will kill the 

 mycelium of the fungus. Immunity and susceptibility (which, 

 however, seem to be always relative only and not absolute) have 

 been proved to be inherited, and in fact to be Mendelian 



