In addition to these fundamentals, there are numerous extensions and
corollaries, some of them of a highly speculative nature. The reader who
is interested in pursuing the subject farther must turn to one of the
text-books on Mendelism.
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.
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