others have produced a single flower; and in
Pennsylvania they ripen their seed six weeks earlier than the tall
maize. Metzger also mentions a European maize which ripens its seed
four weeks earlier than another European kind. With these facts, so
plainly showing inherited acclimatisation, we may readily believe Kalm,
who states that in North America maize and some other plants have
gradually been cultivated further and further northward. All writers
agree that to keep the varieties of maize pure they must be planted
separately so that they shall not cross.

The effects of the climate of Europe on the American varieties is
highly remarkable. Metzger obtained seed from various parts of America,
and cultivated several kinds in Germany. I will give an abstract of the
changes observed[578] in one case, namely, with a tall kind
(Breit-korniger mays, Zea altissima) brought from the warmer parts of
America. During the first year the plants were twelve feet high, and
few seeds were perfected; the lower seeds in the ear kept true to their
proper form, but the upper seeds became slightly changed. In the second
generation the plants were from nine to ten feet in height, and ripened
their seed better; the depression on the outer side of the seed had
almost disappeared, and the original beautiful white colour had become
duskier. Some of the seeds had even become yellow, and in their now
rounded form they approached common European maize. In the third
generation nearly all resemblance to the original and very distinct
American parent-form was lost. In the sixth generation this maize
perfectly resembled a European variety, described as the second
sub-variety of the fifth race. When Metzger published his book, this
variety was still cultivated near Heidelberg, and could be
distinguished from the common kind only by a somewhat more vigorous
growth. Analogous results were obtained by the cultivation of another
American race, the "white-tooth corn," in which the tooth nearly
disappeared even in the second generation. A third race, the
"chicken-corn," did not undergo so great a change, but the seeds became
less polished and pellucid.

These facts afford the most remarkable instance known to me of the direct
and prompt action of climate on a plant. It might {323} have been expected
that the tallness of the stem, th