232 APPENDIX. [No. XV.B. 



maggot of the already punctured aphis ; and thus we have an aphis with a 

 maggot within its body, eating it up, and lastly a maggot within this 

 maggot devouring that also ; in part verification of the lines : 



* 4 Great fleas and little fleas have smaller fleas to bite 'em, 

 The smaller fleas have lesser fleas, so on ad infinitum" 



Aphides live on all plants ; and therefore, if not kept in check, are 

 competent to destroy all human food. They are materially checked by a 

 variety of birds as well as the different insects, such as the swallow, the 

 robin, &c., and also ducks: these destroy them in great numbers; and 

 consider how many aphides it must take to make a meal for a single bird. 

 When a schoolboy I formed the idea of examining the crops of birds, and 

 have found them to contain innumerable insects, and, no doubt, aphides. 

 It must be recommended, then, to protect such birds during the inclemency 

 of winter, and they will amply repay the trouble in summer. Almost every 

 plant has one particular aphis belonging to it, which it has been usual to 

 name according to the plant it infested ; but the aphis which comes more 

 especially under our consideration as being the cause of the potato disease 

 feeds also on a great variety of other plants, both wild and cultivated, 

 many of which are necessary to man for food or medicine, as the wheat, 

 Indian corn, artichoke, turnip, parsnip, shepherd's-purse, mustard, spinach, 

 nightshade, henbane, stramonium, carrot, pasture-grass, couch-grass, 

 spurge, groundsel, celery, &c. &c. ; and every day I am adding to a much 

 longer list fresh specimens attacked by this particular aphis. A dis- 

 tinguished naturalist asked me the other day how I managed to breed 

 aphides. My answer was that I could not help breeding them, for where 

 I do not want them the creatures get upon my sound plants, which they 

 would destroy if not removed. 



The potato disease and failure in crop is no novelty. "We can 

 trace the same disease through a series of years, sometimes occurring 

 here, sometimes there; and the only difference between these visita- 

 tions and the present one is, that this is more general, affecting all 

 localities. Hollins describes a similar disease as occurring about the 

 beginning of the eighteenth century, and states that the Society of Arts 

 awarded a premium for the best remedy. When Martius wrote in the 

 year 1830, the same disease existed and spread rapidly in Germany. This 

 disease is in the potato itself without any difference of opinion; and it 

 is essentially gangrene or death, gangrene of the leaf, of the stem, or 

 different parts of the stem, the underground stem, tuber-bearing stems, 

 and of the different tubers. There are two forms of mortification or 

 gangrene, and they present themselves as the gangrene humide or moist 

 gangrene, and the gangrene sicca or dry gangrene. ,In the moist gangrene 

 the potato-tuber, &c., becomes disorganized by rotting, and is wet and 

 offensive. In the dry gangrene the tuber shrinks up and becomes quite 

 hard and desiccated ; and in certain cases it becomes as hard and dry as a 

 bit of wood, and will even bear a polish. Both kinds of gangrene are 

 accompanied by fungi, but especially the moist ; and all gangrenous tissue 

 is deficient in starch, as may be seen in my preparations under the micro- 

 scope; and examples of the empty condition of the starch cells were 

 figured by Martius when he wrote. Chemical analysis also proves that the 



