SPREADING OF DISEASE AND EPIDEMICS. 145 



certain of the soil fungi -e.g. Agariais inelleus, 

 Deniatophora, etc.^the active creeping forward 

 by growth in the soil of their rhizomorphs and 

 mycelial strands afford examples of active spread- 

 ing of considerable importance in the vineyard 

 and forest, since they pass from root to root and 

 from tree to tree and may infect the entire area 

 in course of time. 



Not the least significant mode of dissemination 

 is that by which what I have termed " lurking 

 parasites " are spread : such are fungi which attach 

 themselves to the seeds, fruits, tubers, etc., of other 

 plants and so obtain all the advantages of being 

 carried and sown with the latter e.g. Ustilagineae 

 and Uredineae which adhere to grain, Verticil- 

 liuiii, Nertria, etc., in potatoes and other plants. 



The spread of diseases due to animals, especially 

 insects, is of course more active, in consequence 

 of the motility of the distributing agents. This 

 is most marked in the winged species, of which 

 locusts, beetles, moths and butterflies, flies and 

 wasps furnish well-known examples ; and is not 

 inconsiderable in the case of wingless and merely 

 creeping species. It is noteworthy that many 

 forms wingless in the parasitic stage are winged 

 at certain periods, e.g. the females of Phylloxera. 



That man also spreads insect pests is well 

 known and acted upon, as witness the phylloxera 

 laws which, however, it is to be feared too often 

 only illustrate once more the adage concerning 

 the shutting of the stable door after the horse 

 has gone. 



K 



