ism.

In plant-breeding a good deal of progress has been made in the exact
study of Mendelian heredity; in animal breeding, somewhat less; in human
heredity, very little. The reason is obvious: that experiments can not
be made in man, and students must depend on the results of such matings
as they can find; that only a very few offspring result from each
mating; and that generations are so long that no one observer can have
more than a few under his eyes. These difficulties make Mendelian
research in man a very slow and uncertain matter.

Altogether, it is probable that something like a hundred characters in
man have been pointed out as inherited in Mendelian fashion. A large
part of these are pathological conditions or rare abnormalities.

But the present writers can not accept most of these cases. It has been
pointed out in Chapter V that there are good reasons for doubting that
feeble-mindedness is inherited in a simple Mendelian fashion, although
it is widely accepted as such. We can not help feeling that in most
cases heredity in man is being made to appear much simpler than it
really is; and that particularly in mental characters, analysis of
traits has by no means reached the bottom.

If we were asked to make out a list of characters, as to the Mendelian
inheritance of which there could be little doubt, we would hardly be
able to go farther than the following:

The sex-linked characters (one kind of color-blindness, hemophilia, one
kind of night-blindness, atrophy of the optic nerve, and a few other
rare abnormalities).

Albinism. This appears to be a recessive, but probably involves multiple
allelomorphs in man, as in other animals.

Brachydactyly, apparently a dominant. This is so much cited in
text-books on Mendelism that the student might think it is a common
character. As a fact, it is extremely rare, being found in only a few
families. The similar trait of orthodactyly or symphalangism, which
likewise appears to be a good Mendelian dominant, seems to exist in only
one family. Traits like these, which are easily defined and occur very
rarely, make up a large part of the cases of probably Mendelian
heredity. They are little more than curiosities, their rarity and
abnormal nature depriving them of evolutionary significance other than
to demonstrate that Mendelian heredity does operate in man.

White blaze in the hair or, as it might better be called to show its
resemblance to the trait found in other