CONTAGIOUS DISEASES OF DOMESTICATED ANIMALS. 249 



pick out the blades of grass they want, and to refnse what they do not 

 like, or whether their young organism is better adapted to resist the in- 

 fluence of the pathogenic principle, I will not now decide, and will only 

 mention that some young animals, even calves, contract the disease in 

 just as acute and severe a form as full-grown cattle. 



6. In the North — say north of the southern boundary line of Kansas — 

 the disease is only communicated through trails, pastures, and grazing 

 grouiuls, or rather their grasses and other food-plants, and water holes 

 previously infected by Southern cattle ; but it usually does not make its 

 appearance until the latter part of July or in August, or until the North- 

 ern prairies, fields, and pastures, owing to the heat and often abundant 

 rains of the summer, contain a comparatively large amount of vegeta 

 ble debris or decaying vegetation, which, it seems, is an important fac- 

 tor in propagating the pathogenic principle if once deposited. That a 

 proi)agation of the once deposited pathogenic principle actually takes 

 place on the grass or herbage of the trails, pastures, or grounds, &c., and 

 outside of the animal organism, is demonstrated by the fact that the 

 period of incubation, as a rule, is a long one, if the native Northern 

 cattle immediately, or within a few daj^s, follow the Southerners on the 

 trails, pastures, &c.; while it usually is considerably shortened if a few 

 or several weeks intervene between the time at which the Southern cat- 

 tle left and the time at which the Northern cattle entered the infected 

 premises. As, however, the infectious principle is not volatile, and is 

 uot disseminated through the air or by winds, its propagation on the 

 grass and herbage of the infested grounds may not be the sole cause of 

 shortening the period of incubation, and the difference just stated may 

 also, to a certain extent, be accounted for by the following fact : In about 

 two, three, or four weeks after a herd of cattle has left its grazing grounds 

 (trail, pasture, prairie, &c., as the case may be) a fine crop of young 

 and juicy grass will be found, if the season is not unfavorable to its 

 growth, wherever the cattle have grazed ; while at all those places or 

 spots where they have not been grazing the grass will be comparatively 

 old and tough. If a herd of native or Northern cattle immediately fol- 

 lows a herd of Texas or other Southern cattle, which have infected the 

 premises with the pathogenic principle of southern cattle fever, the 

 former will principally graze where they find grass, and not where the 

 Southern cattle have cropped it, and where they, at the same time, have 

 deposited, as I shall explain further on, the infectious principle. But if 

 the herd of Northern cattle enters the pastures, &c., formerly- occupied 

 by the Southern cattle two, three, or four weeks after the latter left 

 them, or after a new crop of young grass has made its appearance, the 

 former, for obvious reasons, will prefer to graze at the very places where 

 the Southern cattle have grazed, and deposited the pathogenic princi- 

 ple. As it is well known that the length of the i)eriod of incubation 

 depeiuls, to a certain extent at least, upon the quantity and intensity of 

 the infectious i)rinciple taken up by the animal organism, no further ex- 

 jilanatiou will be necessary. 



