.ON THE SEXUAL ORGANS OF THE CYCADACES. 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, uncon- 
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, Orthoptera, and Neuroptera lived. These are man- 
dibulate 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.* 
ilization? ‘The separation of M xes exists in all the low 
fcil kingdom spent with + and has held to this eee ter in "d 
t periods. Herm ires p^ apih Pien established, and sers oid it 
exists at present but rarely. See maphroditism, i in its perfect form 
brand, 
HS the Upper Chalk of Aix-la- A is acras to be the — formation 
in which Angiosperms have been found. Am them spec ercus, 
Ficus, Juglans, and of several ean Manes qire xty "n seventy species 
of Proteaceae, have ascertained by Dr. Debe ell, *E 
s s the proportion of 1 Pu is nearly the same as in the e vege- 
tation of our own times (Lyell e), it is h hardly possible to o egard these re- 
_ as fixing t hd wer limit Lo: range gio And the 
ra may have still more ee In our own PORK "vegetation, 
