352 
ROT IN SHEEP. 
entozoa in one of their forms, or in a particular stage of 
their development. Now, let us suppose we have the cercaria 
rolled up into a form like a chrysalis covered over by its 
shell; let us also suppose that, which is positively the fact, 
that hundreds of these creatures can be seen upon small mol¬ 
luscs, and many of them also loose in water—that they are 
adhering likewise to plants growing in damp situations; then 
a sheep has only to drink a mouthful of water to take in an 
infinite number of these creatures, and, taking them into 
the stomach, they there become developed into flukes. We 
have, therefore, very good ground for believing that rot can 
be very readily and rapidly received. 
There are certain difficulties which people put before us 
with regard to this theory. They ask how it is that we get 
rot in certain seasons and not in others. Well, we show 
that whenever there is an excess of moisture—whenever, 
in fact, we have great rain-falls—certain pastures and places, 
which are perfectly free from the disease at other times, now 
become affected. This is easily enough explained. We have 
seen that the ova of these creatures come out in millions 
from one affected sheep ; what, then, must be the number 
of them in hundreds of sheep so affected! Now, by direct 
experiment, I have proved that you may take the ova of flukes, 
and keep them upwards of a year before there is any change 
taking place in them. We know not, therefore, what is the 
duration of the life of the ova. It is just possible that they 
may remain, not only one year, but two or more, without 
undergoing any change—not, indeed, until placed under 
favorable circumstances to undergo that change. If, then, 
we look at the vast numbers in which these ova exist, and at 
their power of maintaining their vitality for a great length 
of time, we get rid of a considerable amount of difficulty. 
We find that when we have an excess of moisture, and with 
that an elevated temperature, the living germs, if I may so 
call them, that are now separated by the bursting of the 
operculum of the egg, are set at liberty and become parasitic 
to other creatures, passing through the series of trans¬ 
formations comparatively quickly, and being at length very 
easily received into the system of the sheep. 
I have referred to an elevated temperature combined with 
excessive moisture ; and what is the practical observation 
which is made with reference, for instance, to watery mea¬ 
dows ? It is notorious that sheep placed upon water meadows 
receive the rot; but it is equally notorious that it is only at a 
certain period of the year that they will do so. You may, as 
