8o The Scottish Naturalist. 



ent form. It is long since the belief gained ground among farmers 

 in some parts of England that there was some connection between 

 the abundance of Barberry bushes in any locality, and the preval- 

 ence of disease in wheat in the same locality ; but it was only at a 

 comparatively recent period that the suggestion was offered of a 

 possible genetic relationship between sEcidium Berberidis, the 

 fungus that causes the orange spots on Barberry leaves, and 

 Puccinia graminis (the " black rust ")-, and Uredo linearis (the 

 " red rust "), of wheat. 



That such a connection should exist at first sight appears impro- 

 bable, and for its acceptance ample proof would require to be ad- 

 vanced, — the result of continued and careful experiments under 

 conditions such as to exclude the possibility of error. Yet, how- 

 ever improbable it may appear at first sight, it must be borne in 

 mind that an analogous course of development is undergone by 

 the animal parasites belonging to the group of Entozoa or para- 

 sitic worms ; many of which in the course of their life-history 

 require to pass from the body of one animal into that of another,, 

 as in the case of the Echinococcus of "hydatids" in the human 

 liver, which developes into Taenia Echinococcus in the intestine of 

 the dog; or the still more remarkable development of the " liver- 

 fluke" of the sheep (Distoma hepaticwn), which passes part of its 

 cycle of development in the body of a pond-snail (Limncea trun- 

 catula). A somewhat analogous condition is met with among the 

 insects of the group of Aphides. Brachycolus Stellarice, among 

 the Aphides of Scotland, affords a good example of this change of 

 food-plants, as it may be found in early summer on Caryo- 

 phyllaceous plants (Stellaria, Cerastium, &c), but later in the year 

 it lives on Holcns and other grasses ; on all the plants attacked by 

 it causing pseudo-galls composed of imbricated crowded fleshy 

 leaves. Such facts may well induce caution in rejecting the theory 

 of the Hetercecism of the Uredines without full inquiry. 



Such caution is seen to be needful also if we remember the fre- 

 quency with which the JEcidia are associated with Uredo and 

 Puccinia when, as often occurs, all these forms of parasite are met 

 with on a single host-plant, an association so close in some cases 

 that they occur in the same spot. In such cases the inference 

 would seem natural that there is a genetic relationship between 

 the different forms of fungi. But in these cases in which only one 

 of the forms is known to occur on a host-plant, and where conse- 

 quently the cycle, if such is gone through, must be passed through 

 in part on a different plant, the difficulty of following the develop- 

 ment through all its stages is very greatly increased ; and the evi- 

 dence required to prove the existence of the cycle must be 

 stronger. The necessary proofs can be obtained only by careful, 

 repeated, and continued experiments, from which, to the utmost 

 extent possible, the risk of error has been eliminated. 



The subject has met with far greater attention, and the experi- 

 mental method has been far more largely followed, by Continental 



