sexual cells as compared with
those obtained by the introduction of new blood. The answer to this
question must, moreover, possess the greatest significance for the
correct understanding of sexual reproduction in general.

We see, therefore, that the problems which Darwin had before him in his
researches into the biology of flowers were of the greatest importance,
and at the same time that the point of view from which he attacked the
problems was essentially a teleological one.

We may next inquire in what condition he found the biology of flowers at
the time of his first researches, which were undertaken about the year
1838. In his autobiography he writes,--"During the summer of 1839,
and, I believe, during the previous summer, I was led to attend to the
cross-fertilisation of flowers by the aid of insects, from having come
to the conclusion in my speculations on the origin of species, that
crossing played an important part in keeping specific forms constant."
("The Life and Letters of Charles Darwin", Vol. I. page 90, London,
1888.) In 1841 he became acquainted with Sprengel's work: his researches
into the biology of flowers were thus continued for about forty years.

It is obvious that there could only be a biology of flowers after it had
been demonstrated that the formation of seeds and fruit in the flower
is dependent on pollination and subsequent fertilisation. This proof
was supplied at the end of the seventeenth century by R.J. Camerarius
(1665-1721). He showed that normally seeds and fruits are developed only
when the pollen reaches the stigma. The manner in which this happens was
first thoroughly investigated by J.G. Kolreuter (1733-1806 (Kolreuter,
"Vorlaufige Nachricht von einigen das Geschlecht der Planzen
betreffenden Versuchen und Beobachtungen", Leipzig, 1761; with three
supplements, 1763-66. Also, "Mem. de l'acad. St Petersbourg", Vol. XV.
1809.)), the same observer to whom we owe the earliest experiments in
hybridisation of real scientific interest. Kolreuter mentioned that
pollen may be carried from one flower to another partly by wind and
partly by insects. But he held the view, and that was, indeed, the
natural assumption, that self-fertilisation usually occurs in a flower,
in other words that the pollen of a flower reaches the stigma of
the same flower. He demonstrated, however, certain cases in which
cross-pollination occurs, that is in which the pollen of another flower
of the same species is