It occurs more abundantly, however, in piles of horse manure which have stood for some time in barn yards, or very often in stalls where the manure is allowed to accumulate, has been thoroughly tramped down and then has been left in this condition for some time. It occurs also in composts, hothouse beds, or wherever accumulations of horse manure are likely to occur, if other conditions are congenial. The origin of the natural spawn under these conditions of environment is probably accounted for in many cases by the presence of the spores which have been in the food eaten by the horse, have passed through the alimentary canal and are thus distributed through the dung. The spores present in the food of the horse may be due to various conditions. Horses which go out to pasture are likely to take in with the food obtained in grazing the spores scattered around on the grass, and in the upper part of the sod, coming from mushrooms which grew in the field. In other cases, the spores may be present in the hay, having been carried by the wind from adjacent fields, if not from those which have grown in the meadow. In like manner they may be present in the oats which have been fed to the horse. In the case of stable-fed animals, the inoculation of the manure in this way may not always be certain or very free. But in the case of pasture-fed horses which are stalled at night probably the inoculation is very certain and very abundant, so that a large number of spores would be present in the manure from horses fed in this way. The natural spawn also may originate from spores which are carried by the wind from the pasture or meadow mushrooms upon manure piles, or especially from spores which may lodge in the dust of the highways or street. Many of these spores would cling to the hoofs of the horses and at night, or at times of feeding, would be left with the manure in the stall. At other times horse droppings may be gathered from roads or streets where spores may be present in the dust. The piles of the droppings accumulated in this way, if left a sufficient time, may provide natural spawn by this accidental inoculation from the spores. Probably few attempts have been made to grow the natural spawn with certainty in this country, though it does not appear to be an impracticable thing to do, since formerly this was one source of the virgin spawn in Europe. It is usually obtained by search through stables and barn yards or other places where piles of horse manure have accumulated and have remained for several months. In some cases the growers keep men employed through the summer season searching the yards and stables over a considerable area for the purpose of finding and gathering this natural spawn. It is probably termed virgin spawn because of its origin under these natural conditions, and never having been propagated artificially.