[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.
[19] American Naturalist, vol. 3, p. 45, March, 1869.
[20] See Prof. Torell's discovery of Eophyton Linnæanum, 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, Perlæ, 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 most prolific vegetable growth. * * * Whether the vegetation of the Laurentian was wholly aquatic or in part terrestrial we have no means of knowing." In 1855, Dr. T. Sterry Hunt asserted "that the presence of iron ores, not less than that of graphite, points to the existence of organic life even during the Laurentian or so-called Azoic period." In 1861 he went farther and stated his belief in "the existence of an abundant vegetation during the Laurentian period." The Eophyton in Labrador occurs above the Trilobite (Paradoxides) beds, while in Sweden they occur below.
[21] In a communication made to the Boston Society of Natural History, Oct. 17, 1870 (see also "American Naturalist" for Feb. and Sept., 1871).
[22] On the Origin of Insects, a paper read before the Linnæan Society of London Nov. 2, 1871, and reported in abstract in "Nature," Nov. 9, 1871.
[23] This reminds us (though Ganin does not mention it) of the development of the embryo of Julus, the Thousand legs, which, according to Newport, hatches the 25th day after the egg is laid. At this period the embryo is partially organized, having faint traces of segments, and is still enveloped in its embryonal membranes and retains its connection with the shell. In this condition it remains for seventeen days, when it throws off its embryonal membrane, and becomes detached from the shell.
[24] It is a suggestive fact that these deciduous forms give way through histolysis to true larval forms, just as in some flies (Musca vomitoria) the true larval form goes under, and the adult form is built up from the imaginal disks of the larva. In an analogous manner the deciduous, pluteus-condition of the young Echinoderm perishes and is absorbed by the growing body of the permanent adult stage. This deciduous stage of the ichneumon may accordingly be termed the prelarval stage. Now as we find insects with and without this prelarval stage, and in the radiates quite different degrees of metamorphoses, the inquiry arises how far these differences are correlated with, and consequently dependent upon, the physical surroundings of these animals in the free swimming condition. Merely to point out the differences in the mode of development of animals is an interesting matter, and one could do worse things, but the philosophical naturalist cannot rest here. He must seek how these differences were brought about.
[25] Leuckart, in his great work, "Die Menschlichen Parasiten," p. 700, after the analogy of Hirudo, which develops a primitive streak late in larval life, ventures to consider the first indications of the germ of Nemertes in its larval, Pilidium form as a primitive streak. He also suggests that the development of the later larval forms of the Echinoderms is the same in kind.