diseased
condition to the offspring. Such a transference of microbes is believed
to occur in syphilis and tuberculosis, and has been ascertained to occur
in the case of the muscardine silkworm disease.[216]

_The Theory of Instinct._

The theory now briefly outlined cannot be said to be proved, but it
commends itself to many physiologists as being inherently probable, and
as furnishing a good working hypothesis till displaced by a better. We
cannot, therefore, accept any arguments against the agency of natural
selection which are based upon the opposite and equally unproved theory
that acquired characters are inherited; and as this applies to the whole
school of what may be termed Neo-Lamarckians, their speculations cease
to have any weight.

The same remark applies to the popular theory of instincts as being
inherited habits; though Darwin gave very little weight to this, but
derived almost all instincts from spontaneous useful variations which,
like other spontaneous variations, are of course inherited. At first
sight it appears as if the acquired habits of our trained
dogs--pointers, retrievers, etc.--are certainly inherited; but this need
not be the case, because there must be some structural or psychical
peculiarities, such as modifications in the attachments of muscles,
increased delicacy of smell or sight, or peculiar likes and dislikes,
which are inherited; and from these, peculiar habits follow as a natural
consequence, or are easily acquired. Now, as selection has been
constantly at work in improving all our domestic animals, we have
unconsciously modified the structure, while preserving only those
animals which best served our purpose in their peculiar faculties,
instincts, or habits.

Much of the mystery of instinct arises from the persistent refusal to
recognise the agency of imitation, memory, observation, and reason as
often forming part of it. Yet there is ample evidence that such agency
must be taken into account. Both Wilson and Leroy state that young birds
build inferior nests to old ones, and the latter author observes that
the best nests are made by birds whose young remain longest in the nest.
So, migration is now well ascertained to be effected by means of vision,
long flights being made on bright moonlight nights when the birds fly
very high, while on cloudy nights they fly low, and then often lose
their way. Thousands annually fly out to sea and perish, showing that
the instinct to migrat