The Zoea is born with eight pairs of jointed appendages
belonging to the head, and with no thoracic limbs, while in insects
there are but four pairs of cephalic appendages and three pairs of legs.
Correlated with this difference is the entirely different mode of
grouping the body segments, the head and thorax being united into one
region in the crab, but separate in the insects, the body being as a
rule divided into a head, thorax and abdomen, while these regions are
much less distinctly marked in the crabs, and liable in the different
orders to great variations. The great differences between the Crustacea
and insects are noticeable at an early period in the egg.]

[Footnote 18: Considerations on the Transmutation of Insects in the
Sense of the Theory of Descent. Read before the Imperial
Zoological-botanical Society in Vienna, April 3, 1869.]

[Footnote 19: American Naturalist, vol. 3, p. 45, March, 1869.]

[Footnote 20: See Prof. Torell's discovery of Eophyton Linnaeanum, a
supposed land plant allied to the rushes and grasses of our day, in
certain Swedish rocks of Lower Cambrian age. The writer has, through the
kindness of Prof. Torell, seen specimens of these plants in the Museum
of the Geological Survey at Stockholm. Mr. Murray, of the Canadian
Geological Survey, was the first to discover in America (Labrador,
Straits of Belle Isle) this same genus of plants. They are described and
figured by Mr. Billings, who speaks of them as "slender, cylindrical,
straight, reed-like plants," in the "Canadian Naturalist" for August,
1872.

Should the terrestrial nature of these plants be established on farther
evidence, then we are warranted in supposing that there were isolated
patches of land in the Cambrian or Primordial period, and if there was
land there must have been bodies of fresh water, hence there may have
been both terrestrial and aquatic insects, possibly of forms like the
Podurids, May flies, Perlae, mites and Pauropus of the present day. There
was at any rate land in the Upper Silurian period, as Dr. J. W. Dawson
describes land plants (Psilophyton) from the Lower Heiderberg Rocks of
Gaspe, New Brunswick, corresponding in age with the Ludlow rocks of
England.

We might also state in this connection that Dr. Dawson, the eminent
fossil botanist of Montreal, concludes from the immense masses of carbon
in the form of graphite in the Laurentian rocks of Canada, that "the
Laurentian period was probably an age of mos