ualities. There is often no obvious peculiarity of
structure, or appearance, indicating the possession of diseases or
defects which are transmissible, and so, special care and continued
acquaintance are necessary in order to be assured of their absence in
breeding animals; but such a tendency although invisible or
inappreciable to cursory observation, must still, judging from its
effects, have as real and certain an existence, as any peculiarity of
form or color.

Every one who believes that a disease may be hereditary at all, must
admit that certain individuals possess certain tendencies which render
them especially liable to certain diseases, as consumption or
scrofula; yet it is not easy to say precisely in what this
predisposition consists. It seems probable, however, that it may be
due either to some want of harmony between different organs, some
faulty formation or combination of parts, or to some peculiar physical
or chemical condition of the blood or tissues; and that this altered
state, constituting the inherent congenital tendency to the disease,
is duly transmitted from parent to offspring like any other quality
more readily apparent to observation.

Hereditary diseases exhibit certain eminently characteristic
phenomena, which a late writer[2] enumerates as follows:

1. "They are transmitted by the male as well as by the female
parent, and are doubly severe in the offspring of parents both
of which are affected by them.

2. They develop themselves not only in the immediate progeny of
one affected by them, but also in many subsequent generations.

3. They do not, however, always appear in each generation in the
same form; one disease is sometimes substituted for another,
analogous to it, and this again after some generations becomes
changed into that to which the breed was originally liable--as
phthisis (consumption) and dysentery. Thus, a stock of cattle
previously subject to phthisis, sometimes become affected for
several generations with dysentery to the exclusion of phthisis,
but by and by, dysentery disappears to give place to phthisis.

4. Hereditary diseases occur to a certain extent independently
of external circumstances; appearing under all sorts of
management, and being little affected by changes of locality,
separation from diseased stock, or such causes as modify the
production of non-hereditary diseases.

5. They