of hereditary characters; that is to say paternal or
maternal hereditary engrams.

(2). No individual is absolutely identical with another.

(3). On the average, each individual resembles more especially its
direct ancestry and its parents, and differs more markedly from its
parentage the more this is remote.

We shall see later on that the ancestral relationship of the different
groups, species and varieties of animals has been fairly well fixed,
and we may say that the third of the laws stated above is equally true
in a wider sense. In fact the species and varieties of animals which
are near related resemble each other, while the _genera_, families
and classes are more dissimilar as their relationship is more remote.
We employ here the terms resemblance, homology and difference in their
profound and general sense. Certain purely external resemblances, due
to phenomena of convergence, must not be considered as homologies in
the sense of hereditary relationship. Thus, in the language of natural
history we do not say that a bat resembles a bird, nor that a whale
resembles a fish, for here the resemblances are due simply to aerial
or aquatic life which produces the effects of convergence, while the
internal structure shows them to be quite dissimilar organisms.
Although it swims in the sea the whale is a mammal; its fins at first
sight resemble those of a fish, but they are really the homologues of
the four limbs of other mammals and contain the corresponding bones.

In man, we see that brothers and sisters resemble each other in a
general way, but that each one is dissimilar in some respects from the
others. If we compare different families with many children we find
that brothers and sisters resemble each other the more their parents
are alike and come from a uniform ancestry which has undergone little
crossing, while the crossing of different races and human varieties
results in the production of individuals which differ from each other
considerably, even when they come from the same couple.

If we examine things more closely, we find that the characters of each
of the offspring of the same couple present neither simple repetition
nor an equal mixture of the peculiarities of the parents, but very
diverse combinations of the characters of several ancestors. For
instance, children may bear a striking resemblance to a paternal
grandfather, a maternal grand-aunt, or a maternal great-grandmother,
etc. This is called _