ritive tissue, the endosperm.), the structure of
the stamens, and the arrangement of the parts of the flower. All these
points are common to Monocotyledons and Dicotyledons, and separate the
Angiosperms collectively from all other plants.

In geological history the Angiosperms first appear in the Lower
Cretaceous, and by Upper Cretaceous times had already swamped all other
vegetation and seized the dominant position which they still hold. Thus
they are isolated structurally from the rest of the Vegetable Kingdom,
while historically they suddenly appear, almost in full force, and
apparently without intermediaries with other groups. To quote Darwin's
vigorous words: "The rapid development, as far as we can judge, of
all the higher plants within recent geological times is an abominable
mystery." ("More Letters of Charles Darwin", Vol. II. page 20, letter
to J.D. Hooker, 1879.) A couple of years later he made a bold suggestion
(which he only called an "idle thought") to meet this difficulty. He
says: "I have been so astonished at the apparently sudden coming in of
the higher phanerogams, that I have sometimes fancied that development
might have slowly gone on for an immense period in some isolated
continent or large island, perhaps near the South Pole." (Ibid, page
26, letter to Hooker, 1881.) This idea of an Angiospermous invasion from
some lost southern land has sometimes been revived since, but has not,
so far as the writer is aware, been supported by evidence. Light on the
problem has come from a different direction.

The immense development of plants with the habit of Cycads, during the
Mesozoic Period up to the Lower Cretaceous, has long been known. The
existing Order Cycadaceae is a small family, with 9 genera and perhaps
100 species, occurring in the tropical and sub-tropical zones of both
the Old and New World, but nowhere forming a dominant feature in the
vegetation. Some few attain the stature of small trees, while in the
majority the stem is short, though often living to a great age. The
large pinnate or rarely bipinnate leaves give the Cycads a superficial
resemblance in habit to Palms. Recent Cycads are dioecious; throughout
the family the male fructification is in the form of a cone, each scale
of the cone representing a stamen, and bearing on its lower surface
numerous pollen-sacs, grouped in sori like the sporangia of Ferns. In
all the genera, except Cycas itself, the female fructifications are
likewise co