hough invisible or
inappreciable to careless observers, must still, judging from its
effects, have as real and certain an existence as any peculiarity of
form or color.

In neat cattle, hereditary diseases do not usually show themselves at
birth; and sometimes the tendency remains latent for many years, perhaps
through one or two generations, and afterward breaks out with all its
former severity. The diseases which are found hereditary in cattle are
scrofula, consumption, dysentery, diarrhoea, rheumatism, and malignant
tumors. As these animals are less exposed to the exciting causes of
disease, and less liable to be overtasked or subjected to violent
changes of temperature, or otherwise put in jeopardy, their diseases are
not so numerous as those of the horse, and what they have are less
violent, and generally of a chronic character.

With regard to hereditary diseases, it is eminently true that "an ounce
of prevention is worth a pound of cure." As a general and almost
invariable rule, animals possessing either defects or a tendency to
disease, should not be employed for breeding. If, however, for special
reasons it seems desirable to breed from one which has some slight
defect of symmetry, or a faint tendency to disease--although for the
latter it is doubtful whether the possession of any good qualities can
fully compensate--it should be mated with one which excels in every
respect in which it is itself deficient, and on no account with one
which is near of kin to it.

There is another law, by which that of similarity is greatly
modified--the law of _Variation_ or divergence.

All animals possess a certain flexibility or pliancy of organization,
which renders them capable of change to a greater or less extent. When
in a state of nature, variations are comparatively slow and infrequent;
but when in a state of domestication they occur much oftener and to a
much greater extent. The greater variability in the latter case is
doubtless owing, in some measure, to our domestic productions' being
reared under conditions of life not so uniform as, and different from,
those to which the parent species was exposed in a state of nature.

Among what are usually reckoned the more active causes of variation may
be named _climate_, _food_, and _habit_. Animals in a cold climate are
provided with a thicker covering of hair than in warmer ones. Indeed, it
is said that in some of the tropical provinces of South America, there
are ca