PERIODICAL OPHTHALMIA OF HORSES. 
363 
man, that the practical breeders of horses in Yorkshire con¬ 
sidered this complaint in a great measure hereditary ; and for 
that reason they always had a great objection to breeding from 
either a horse or mare that had gone blind : they said it was al¬ 
most sure to “ run in the stock.” I was laughed at, 1 remember, 
for giving utterance to such a vulgar and obsolete crotchet. Many 
of those, however, that were then present imbibing the Profes¬ 
sor's ingenious theories, have since learned to think differently. 
But in general 1 have remarked, that those persons who have be¬ 
gun with theory, and nothing but theory, for their guide; who 
have, in early life, devoted all the energies of their minds to hy¬ 
pothetical reasoning, and are become enthusiasts in this way, 
seldom attain to any considerable proficiency in practical know¬ 
ledge. They seem always striving to pervert facts, to bend and 
twist them to their own preconcerted notions; not recollecting 
that theories are continually changing, or going into the most 
profound oblivion, w hile facts wall remain for ever the same. 
If I remember right, Mr. Coleman used frequently to say, 
“ I know r of no such thing as hereditary disease—strictly speak¬ 
ing’, no disease can be called hereditary.” and, perhaps, in 
a literal sense, this position is tenable. We cannot, however, 
doubt what amounts, I think, to much the same thing,—that 
a strong disposition to particular diseases constantly does run in 
families. It may be very true, that the horse living in a state of 
nature is not at all subject to periodical ophthalmia; and it may 
also be very true, as has been asserted, that if a man born of 
gouty parents w ere to live the life of a savage, or of the wander¬ 
ing Arab of the desert, hewvould never be liable to be attacked 
with gout. Yet this does not alter the every-day fact which is 
constantly impressed upon the most common observer, that, under 
other circumstances, certaiu disorders commonly do descend, 
transferred and entailed, as it w^ere, from father to son, and from 
one generation to another. It only, perhaps, tends to shew, that 
an artificial state of animal existence is the principal, the most 
general, exciting cause of all the maladies that “ life is heir to.” 
Now I care not whether this be called hereditary disease, or only 
an original predisposition; it amounts, I say, to just the same 
thing’: and I venture to affirm, that this is by far the most frequent 
origin of the complaint in question, the periodical ophthalmia of 
horses. But while I assert this, I am ready to admit, that it also 
arises from a variety of other causes, quite adventitious, and un¬ 
connected with this source. 
In a complaint of this nature, which is sometimes two or throe 
years in running its course, a regiment of cavalry, perhaps, 
affords the best opportunity for observation ; where three and 
