160 SCAUIES. 



Gerlach adds some remarks on the classification and on the 

 procreation and development of these parasites; he further 

 alludes to them as the cause of skin diseases, and mentions 

 the facts known with reference to the diffusion of the species 

 and the methods of discovering them in animals. 



" The itch and mange insects," says Gerlach, " are met 

 with in cold and hot climates ; it appears, however, that the 

 itch is most rife in southern regions; but, as regards mange, 

 this has not been established. It is not probable that there 

 is a real geographical limitation in the spread of these para- 

 sites ; their spread depends on so many causes, such as habits, 

 customs, methods of living, state of cultivation, rearing of 

 animals, sanitary laws, and the method in which these are 

 enforced. There are certain forms of parasites imported in 

 a district, where, under certain circumstances, they may 

 readily remain stationary, without genuine development from 

 any determined local condition, depending on the position, 

 or otherwise, of the locality ; in this way there are certain 

 geographical itch and mange stations, and in some territories 

 both may be seen ; whereas in others, as in large cities, in 

 some parts there is one form of insect, and another elsewhere. 

 Of the mange insects, that of the sheep, and nextly that of the 

 horse, are most widely spread. Poland would appear to be a 

 geographical station for all the mite tribe, at least for 

 the mange insects; from there severe invasions of these 

 parasites occur on our land, especially among sheep and 

 horses: in Berlin, the dog-mange is stationary; in West- 

 phalia, the mange in cats ; in France, the scab in sheep ; in 

 Switzerland, the scab amongst goats, which, at all events, in 

 the north of Germany, does not exist. 



" The acari are not immediately and essentially affected by 

 meteorological changes, as they live on living organisms ; we 

 cannot, therefore, as for other insects, speak of productive 



