BOTANTOAL BIOLOGY. 421 



thing is that tljis Bacillus, which can be cultivated externally to the 

 body, if kept at a heightened ten)peiature, can l)e attenuated in its viru- 

 lence. It dro})s in tact the excretion of the poison. It is then found 

 that, if injected into the blood, it does no mischief, and, what is more 

 extraordinary, if the Bac ill its in its most lethal form is subsequently in- 

 troduced, it too has lost its power. The exidanation of the immunity 

 in this case is entirely ditterent from that which was suggested by a 

 consideration of the facts of relapsing fever. The researches of Met- 

 schnikoff have led to the hypothesis that in the present case the white 

 blood-corpuscles destroy the BacillKs, When they tirst coiue into con- 

 tact with these in their virulent form, they are unable to touch them. 

 But if they have been educated by first having presented to them the 

 attenuated form, they find nodifiiculty in grappling with the malignant. 

 This is a very remarkable view. 1 should not have put it before you had 

 there not been solid reasons for regarding the idea of the education of 

 protoplasm with scientific respect. Tiie [)lasmodia of the Myxomycetes, 

 which consist of naked protoplasui, are known to become habituated to 

 food whi(;h they at first reject, and the researches of Beyerinck on the 

 disease known as "gumming'' in plants have apparently shown that 

 healthy cells may be taught, as it were, to produce a ferment which 

 otherwise they would not excrete. 



If Metschnikoff's theory ])e true, we have a rational explanation of 

 vaccination and of preventives inoculation generally. It is probably 

 however not the only explanation. x\nd the theory of the inhibitive 

 action upon itself of the products of the ferment-organism's own activity 

 is still being made the basis of experiment. In fact, the most recent 

 results point to the possibility of obtaining protection by injecting into 

 the blood substances artificially obtained entirely' iudependeut of the 

 organisms whose development they inhibit. 



It is impossible for me to touch on these important matters at any 

 greater length, but I doubt if the theory of fermentation, as applied to 

 the diseases of organisms, has as yet more than opened its first page. 

 It seems to me possible, that besides the rational explanation of zymotic 

 disease, it may throw light on others where owing to abnormal condi- 

 tions, the organism, as in the case of Berard's ])lums, is itself the agent 

 in its own fermentative processes. 



And now I must conclude. I have led you, I am afraid, a too lengthy 

 and varied a journey in the field of botanical study. But to sum up my 

 argument : I believe I have shown you that at the bottom of every 

 great branch of biological inquiry it has never been possible to neglect 

 the study of plants ; nay moie, that the study of plant-life has gener- 

 ally given the key to the true course of investigation. Whether you 

 take the problems of geographical distribution, the most obscure points 

 in the theory of organic evolution, or the innermost secrets of vital 

 l)henomena, whether in health or disease, — not to consider plants is still, 

 in the words of ]\Ir. Darwin, '-a gigantic oversight, for these would 

 simplify the ])roblem." 



