Hereditary Diseases of Sheep and Pigs. 
19 
Cubag^ua with toes half a span in lenj^th, and in many other 
parts of the world witli the hoof divided into four di»Its. The 
celebrated naturalist, Buffon, says that pigs in Guinea " have 
very long ears, couched upon the back ; in China a large 
pendent belly and very short legs ; at Cape Verd and other 
places very large tusks, crooked like the horns of oxen ; in 
domestication half pendent and white ears."* Now all these 
characters are decidedly hereditary, and though, as above men- 
tioned, some of them may, in the first instance, have been 
gradually induced by the continued operation of external cir- 
cumstances, still they have become fixed and permanent, and 
can only be altered or removed by crossing for several succes- 
sive generations with animals possessing different or opposite 
characters, or by the modifying influences produced by change of 
climate, &c. 
The hereditary transmission of defects and diseases is often 
less obvious than that of form, size, and colour ; for tliere is 
sometimes no very apparent peculiarity of structure or general 
appearance in animals well known to possess a decided tendency 
to disease. But such a tendency, though sometimes invisible, 
and even inappreciable to ordinary powers of observation, must 
still, judging from its effects, have as certain and definite an 
existence as any external peculiarity of form. Every one who 
believes that a disease may be hereditary at all must admit that 
certain individuals possess certain peculiarities which render 
them unusually liable to certain diseases, as scrofula, consump- 
tion, and dysentery ; yet no one can say precisely in what this 
special predisposition consists. We believe that in each case it 
consists in some faulty formation, some want of harmony between 
different parts and organs of the body, or some peculiar physical 
or chemical condition of the blood or soft solids ; and that this 
altered state, constituting the inherent congenital tendency to the 
disease, is duly transmitted from parent to offspring, like any of 
the corporeal qualities above referred to. 
From what has just been stated, it will be evident that it is not 
usually the disease, but only the tendency or predisposition to it, 
which is hereditary. Few individuals are born with any par- 
ticular disease already developed, but many with a predisposition 
to disease. Such a predisposition may vary much in degree and 
intensity. It may be so strong that no care or skill can prevent 
the development of disease, or so slight that it docs not interfere 
with the health or usefulness of the animal, or cause actual dis- 
ease, unless with the co-operation of active exciting causes. 
Thus, a predisposition to consumption is the ill-fated inheritance 
* Natural History of Man, by James Prichard, M.D., p. 32. 
c 2 
