740 PARASITIC DISEASES OF ANIMALS. 
ditches, provide an inexhaustible and constant supply of 
new material. Marshy land is not in itself “ rotting land ” 
until it has been fed over by sheep infested with flukes; but 
the chances are that, having once become contaminated, it 
never recovers its healthy condition. 
Healthy lands become rotting lands in wet seasons, as the 
experience of I860 and 1879 distinctly proves; and the fact 
has led to the expression of some doubt as to the true causes 
of rot, because it is alleged the parasites cannot be called 
into existence on pastures where none of the germs are 
present by the mere falling of the rain on the ground, and it 
must be admitted that it is not always easy to explain the 
sudden occurrence of rot under these conditions. It is, 
however, settled beyond all doubt that the disease does arise 
from the ravages of the fluke, and never occur in localities 
in which those parasites are not produced. Wet is an essen¬ 
tial element for the development of the worms, and it must 
therefore be concluded that the germs of the parasites are 
often present in places where they are not suspected, and 
w here they will remain in an undeveloped form, perhaps for 
a long period, unless the conditions are exceptional—that is 
to say, unless they are destroyed by heat and drought, or 
rendered active by long-continued wet, which supplies the 
conditions required for their growth, chiefly by affording the 
appropriate habitat for the soft-bodied creatures (water snails) 
to which they become parasitic in the first place. 
All the varieties of the fluke pass through several genera¬ 
tions before they are sufficiently mature to be introduced into 
the system of a warm-blooded animal, in which position 
they rapidly acquire sexual organs and produce an immense 
quantity of ova. Experiments vdiich have been made with 
the eggs of the fluke prove that they are not capable of 
advancing a single step in the animal organism; they have 
been given in considerable quantities to sheep w ithout causing 
any inconvenience, and the fact is clearly established that 
they must undergo the various changes wffiich occur, from 
the issue of the ciliated embryos to the occurrence of the 
inchoate form of the fluke, outside the body of the higher 
animal. When the flukes in the larval form are introduced 
into the digestive organs of the animals which are feeding 
on the infested pastures, they probably remain for some 
time without giving any sign of their presence, and the 
sheep may be removed to other pastures, at a distance, 
before any indications of rot are apparent; and in such cases 
the ground on which they fed last commonly gets credited 
with the mischief. It is stated, truly enough, that the sheep 
