232 THE OEIGIN AND PROPAGATION OF DISEASE. 



isms. The phenomena of the so-called alternation of generations and of 

 migration from one habitat or locality to another, are by no means con- 

 fined to animal parasites. On the contrary, the most remarkable instances 

 of both are to be found in cryptogamic vegetables. Fungi formerly 

 regarded as distinct species, and even as belonging to different genera- 

 are seen to be successive forms of the same plant, following each other 

 in definite order through the regular cycle of their annual reproduction. 



The three fungi, known as Trichobasis^ Fuccinia, and JEcidium, appear 

 in succession, as different members of the same specific generation, upon 

 the cereal grains in summer and in autumn, and upon the barberry in 

 the spring 5 while corresponding differences are to be seen in their spores 

 and mode of germination at these different epochs. 



It would perhaps be difficult to imagine a scientific pursuit less likely 

 to produce anything of value for practical medicine than the study of 

 microscopic fungi growing as parasites upon other vegetables. And 

 yet, if it should finally turn out that these minute researches are prelim- 

 inary to the discovery of a means for preventing or controlling an epi- 

 demic of scarlatina, we can say with truth that such a result would not 

 be more remarkable than many which have actually followed from purely 

 scientific investigations in chemistry and physics. 



At all events, it is certain that these botanical discoveries have had 

 an important influence in directing medical research in the path which it 

 is now following. It could hardly be otherwise, from the moment they 

 were found to have a direct connection with certain epidemic diseases 

 in the vegetable world, some of which are of great practical conse- 

 quence to us as affecting the annual supply of food. 



Let me remind you of the history of our knowledge in regard to the 

 disease known as the potato-rot. 



This disease first made its appearance, so far as we know, about 

 thirty years ago. The most destructive season of that epidemic in this 

 country was was in 1844.* Previously to that time, the annual crop of 

 potatoes in the United States amounted to over one hundred million 

 bushels ; but, in consequence of the blight, it was reduced in some parts 

 of the country to one-half or even to one-quarter of the ordinary yield.f 



In 1845, it showeditself in England, Scotland, and Ireland, and spread 

 with great rapidity. This is the account of it given by Mr. Cooke,| one 

 of the highest authorities on that subject : 



^'It first apx)eared in the Isle of Wight, in the middle of August; a 

 week afterward it had become general in the south of England, and at 

 the end of a fortnight there were but few sound samples of potatoes in 

 the Loudon market. The course of the disease was this : in the month 

 of July or August the leaves of the vines would be suddenly seen to be 

 marked with black spots. They then began to wither, and give off an 



* American Quarterly Journal of Agriculture and Science, January, 1845. 



t " Patent-Office Eeports, Department of Agriculture," 1856. 



X "Introduction to the Study of Microscopic Fungi," London, 1870, pp. 144, 146. 



