N. I. Vavilov 51 



strictly speaking, reagent) for the recognition of the species and races 

 in systematic and genetic studies of plants. 



The general idea of a connection between immunity ;ind genetics is 

 certainly very conspicuous, and there are many data to prove it. It 

 is enough to mention the well-known fact that nearly allied species 

 of animals are very often susceptible to the same diseases ; nearly 

 related genera of plants very often suffer from the same insects. Several 

 genera of parasitic fungi are exclusively connected with definite families 

 of plants, as for example Phragmidium with the family Rosaceae. 



The general indications of the possibility of applying fungal reactions 

 of plants in ascertaining their affinity, may be found in mycological 

 literature. But, notwithstanding the certainty of a relation between 

 immunity and genetics, very few cases are known of the actual use of 

 fungi or other parasites as tests in genetic and systematic work. There 

 are probably two reasons which explain the neglecting of this method : 

 firstly, because in the majority of cases, fungi are not sufficiently 

 specialized to be employed for the delicate differentiation of nearly 

 related plants, and secondly, on account of the usual divergence of the 

 work of pathologists, genetists and systematists. 



As to examples in literature of the use of parasites as physiological 

 tests, I can cite very few. First, Prof Klcbahn in his monograph on 

 rusts (7), pp. 140 — 141, in the paragraph entitled: " Verwendung der 

 Spezialisierung des Schmarotzers zur Unterscheidung der Arten und 

 Wirte," mentions a case in which by the aid of narrowly specialized 

 rust Melampsora ribesii jMrpureae he found a mistake in the denomina- 

 tion of a willow plant in his garden. A second example occurs in the 

 work of Eriksson entitled : Ein parasitischer Pilz uls Index der inneren 

 Natur eines Pflanzenbastards (4). In this work he has shown that the 

 hybrid of wheat and rye is immune to brown rye-rust Puccinia dispersa 

 and susceptible to brown wheat-rust P. triticina, which proves that this 

 plant is nearer to wheat than to rye (16, p. 104). A third case is furnished 

 by E. M. Vasiliev's observations on the injurious insects of maize in 

 which he makes an attempt to connect the number of species of these 

 insects with the origin of this plant (15)'. 



' In Darwin's Variations of Animah and Plants under Domestication, second edition, 

 Vol. II, Chap. XVIII, we find the following footnote about the affinity of Aperea and 

 guinea-pigB. " I sent to Mr H. Denny, of Leeds, the lice which I collected from the wild 

 Aperea in La Plata, and he informs me that they belong to a genus distinct from those 

 found on the guinea-pig. This is important evidence that the Aperea is not the parent of 

 the guinea-pig; and is worth giving, as some authors erroneously suppose that the guinea- 

 pig since being domesticated has become sterile when crossed with the Aperea. " 



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