CLASSIFICATION 55 



aecidium-stage, and that its spores, lying hid in the grains of 

 corn, germinate when the wheat germinates and infect the 

 new wheat-plant with the fungus ; or else that in Australia 

 and India rust finds other hosts which serve its purpose 

 equally well. There are difficulties still to be cleared up 

 regarding the mode of infection of the corn. If our subject 

 were the botany of parasitic fungi we should have to look 

 further into the matter, but as an illustration of the relation 

 which has been supposed to exist between the germs of 

 cancer in Man and their life in a vegetable host the analogy 

 is sufficiently complete. Can we say of cancer that it does 

 not occur where the suspected tree is absent? On the 

 contrary, cancer is found in coral islands where the coca-palm 

 is the only tree, and on the plains of North America, where 

 no tree raises its trunk for more than a thousand miles. In 

 other districts other conditions have been found associated 

 with great prevalence of cancer ; but as yet none have stood 

 the test of the control experiment. At present, therefore, 

 the concurrence of the tree and cancer, like the concurrence 

 of various other conditions and this fell disease, must be 

 looked upon as a coincidence. 



The control- observation is the key to the position. Para- 

 doxical as it sounds, the ingenuity of the man of science is 

 taxed not in making observations and devising experiments, 

 but in planning how to unmake them. The real difficulty is 

 not experienced in imagining a possible cause for a known 

 effect, but in devising an observation in which the supposed 

 predisposing condition is absent, while all other conditions 

 remain the same. The animal-magnetisers of fifty years ago 

 asserted that their subjects were attracted by certain metals 

 and repelled by others. Braid, to whose scientific investiga- 

 tion of the phenomena of hypnotism we owe the dissipation 

 of numerous errors, when attending one of their seances, 

 asserted with the same confidence, in the presence of the 

 hypnotised person, that he would clutch at a round object 

 and shrink from a pointed one. When he offered him the 



