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ON THE SEXUAL ORGANS OF THE CYCADACE^. 101
In the economy of nature we find numerous and intimate relations
between the vegetable and animal kingdoms, by which they mutually
influence one another. Having recognized the fundamental law that
the chemical compounds which serve to build up the animal structure
have been elaborated by plants, we now see that, on the other hand,
the animal kingdom forms an indispensable condition for the existence
of vegetables. Fertilization, in the majority of cases an essential con-
dition to the reproduction of vegetable species, is usually only pos-
sible among angiospermous plants by means of the intervention of
insects. Where, formerly, it was only seen in isolated cases to which
little importance was attached, modern science has discovered a natural
law. At the same time it has shown that it is especially the Diptera
and Lepidoptera, that is, sucking insects (Haustellata), which, luicon-
scious .fertilizers of plants, perform in nature the important duty of
maintaining the existence of the vegetable kingdom, at least as far as
the higher orders are concerned.
We may also consider this relation in connection with time, and
inquire from what epoch it dates. The researches on fossil insects
which we owe to Germar, Unger, Oswald Heer, and others, have
shown that all the Orders of insects have not appeared simultaneously.
In the Paleozoic epoch, when angiospermous Dicotyledons did not yet
exist, Coleoptera, Orlhoptera, and Newroptera lived. These are man-
dibvdate insects, which do not visit plants for their nectar. The first
Diptera date from the Jurassic epoch, but the appearance in great
numbers of haustellate insects occurs at and after the Cretaceous epoch,
when the plants with pollen and closed carpels (Angiosperms) are
found, and acquire little by little the preponderance in the vegetable
kingdom.*
evolution of organic nature a tendency to arrive at the possibility of this selffertilization
? The separation of the sexes exists in all the lower plants ; the
vegetable kingdom commenced with it, and has held to this character in all
past periods. Hermaphroditism has been established, and physiologically it
exists at present but rarely. See on hermaplu'oditism, in its perfect form,
Hildebrand, 1. c. p. 57.
* The Upper Chalk of Aix-la-Chapelle is stated to be the oldest formation
in which Angiosperms have been found. Among them species of Querctis,
Fictis, Jufflans, and of several Myrtaceous genera, with sixty to seventy species
oi ProteacecB, have been ascertained by Dr. Debey (Lyell, 'Elements of Greology,'
p. 330). As the proportion of Dicotyledons is nearly the same as in the vegetation
of our own times (Lyell, 1. c), it is hardly possible to regard these remains
as fixing the lower limit to the range in time of Angiosperms. And the
Flora may have been still more varied. In our own indigenous vegetation.