in the fashion of the mole, but it is interesting to note that the
skeleton is modified for the same purpose in a manner quite different
from that obtaining in the latter animal. These animals derive their
name from the metallic iridescence of the fur of most of the species.
In the more typical species the dental formula is the same as in
Microgale, that is to say, there are 40 teeth. In other species, which
it has been proposed to separate as Amblysomus, there are, however,
only 36 teeth, owing to the absence of the last pair of molars. The
group is evidently nearly related to the Centetidae—most nearly
perhaps to the Oryzorictinae.
Fossil Insectivora.
Some years ago Dr F. Ameghino, of Buenos Aires, described from the Tertiary formation of Santa Cruz, in Patagonia, the remains of an insectivore under the name of Necrolestes. The occurrence of a member of the Insectivora in these beds is remarkable, since this group is represented at the present day in South America only by a shrew or two which have wandered from the north. Dr Ameghino expressed his belief that the extinct Patagonian insectivore was nearly related to the golden moles, and although this opinion appears to have been withdrawn, Professor W. B. Scott states that he is convinced of the close affinity existing between Necrolestes and Chrysochloris. Although this view may not be accepted, it must be remembered that it represents the opinion of a palaeontologist who has had better opportunities than most of his fellow-workers of forming a trustworthy judgment. So convinced is Dr Scott of the closeness of the relationship between Necrolestes and the golden moles that he regards it as rendering probable the former existence of a direct land-connexion between Africa and South America. There is no reason, he says, to suppose that the track of migration could have been by way of Europe and North America, for no trace of the group has been found anywhere north of the equator. This supposed connexion between Africa and South America in Tertiary times has often been suggested, and is supported by many independent lines of evidence; and the presumed affinity between the two mammals here referred to adds to the weight of such evidence.
The discovery in the Oligocene Tertiary deposits of Dakota of the remains of a species of hedgehog is a fact of great interest, for the hedgehog-tribe (Erinaceidae) is at the present day an exclusively Old World group. The discovery of the fossil American species, which has been made the type of a new genus under the name of Protherix, serves to strengthen the view that the northern countries of the Western and Eastern hemispheres form a single zoological region; and that formerly there was comparatively free communication between them in the neighbourhood of Bering Sea, under climatic conditions which permitted of temperate forms passing from one continent to the other. As might have been expected, remains of hedgehog-like mammals have been obtained in the Tertiary deposits of Europe. Among these, Palaeoerinaceus, from the Upper Oligocene of France, seems scarcely separable from the existing genus. Necrogymnurus (Neurogymnurus) from the Lower Oligocene, of the same country, appears to be allied to Hylomys, which is itself the most generalised of the family, so that the extinct genus, of which Caluxotherium is a synonym, may represent the ancestral type of the Erinaceidae. The genus Galerix, or Lanthanotherium, of the Oligocene, which has the typical series of 44 teeth, a bony ring round the orbit, and conjoint tibia and fibula, has been regarded as representing the Tupaiidae and Macroscelididae, but is more probably referable to the Erinaceidae, being apparently akin to Gymnura. The moles are represented in the French Oligocene by Amphidozotherium and in the Miocene by Talpa, while in the North American early Tertiary we have the primitive Talpavus. Shrews are also known from the Lower Oligocene upwards both in the eastern and western hemispheres. Of the Lower Eocene Adapisorex, with the typical 22 lower teeth, Adapisoriculus and Orthaspidotherium, all from France, the affinities are quite uncertain. The American Oligocene Leptictis, with i. 2, c. 1, p. 4, m. 3 in the upper jaw, and Ictops, with i. 32, c. 11, p. 44, m. 33, may be insectivorous mammals, with affinities to the creodont Carnivora. It is, indeed, probable that not only is there a relationship between the Creodonta and the Insectivora, but also one between the latter and the Marsupialia, so that the marked similarity between the cheek-teeth of the insectivorous Chrysochloris and the Marsupial Notoryctes may be due to genetic relationship. That the bats and the flying-lemur are descendants of the Insectivora cannot be doubted.
Bibliography.—G. E. Dobson, “Monograph of the Insectivora” (London, 1883–1890); W. Leche, “Zur Morphologie des Zahnsystems der Insectivoren,” Anatom. Anzeiger (xiii. 1 and 514, 1897); C. J. Forsyth-Major, “Diagnoses of New Mammals from Madagascar.” Ann. Mag. Nat. Hist. ser. 6. vol. xviii. pp. 318 and 461 (1896): A. A. Mearns, “Descriptions of New Mammals from the Philippine Islands,” Proc. U.S. Museum (xxviii. 425, 1905). (R. L.*)
INSECTIVOROUS PLANTS. Insectivorous or, as they are sometimes more correctly termed, carnivorous plants are, like the parasites, the climbers, or the succulents, a physiological assemblage belonging to a number of distinct natural orders. They agree in the extraordinary habit of adding to the supplies of nitrogenous material afforded them in common with other plants by the soil and atmosphere, by the capture and consumption of insects and other small animals. The curious and varied mechanical arrangements by which these supplies of animal food are obtained and utilized are described under the headings of the more important plants.
The best known and most important order of insectivorous plants—Droseraceae—includes six genera: Byblis, Roridula, Drosera, Drosophyllum, Aldrovanda and Dionaea, of which the last three are monotypic, i.e. include only one species. The Sarraceniaceae contain the genera Sarracenia, Darlingtonia, Heliamphora, while the true pitcher plants or Nepenthaceae consist of the single large genus Nepenthes. These three orders are closely allied and form the series Sarraceniales of the free-petalled section (Choripetalae) of Dicotyledons. The curious pitcher-plant, Cephalotus follicularis, comprises a separate natural order Cephalotaceae, closely allied to the Saxifragaceae. Finally the genera Pinguicula, Utricularia, Genlisea and Polypompholix belong to the gamopetalous order Lentibulariaceae.
While the large genus Drosera has an all but world-wide distribution, its congeners are restricted to well-defined and usually comparatively small areas. Thus Drosophyllum occurs only in Portugal and Morocco, Byblis in tropical Australia, and, although Aldrovanda is found in Queensland, in Bengal and in Europe, a wide distribution explained by its aquatic habit, Dionaea is restricted to a few localities in North and South Carolina. Cephalotus occurs only near Albany in Western Australia, Heliamphora on the Roraima Mountains in Venezuela, Darlingtonia on the Sierra Nevada of California, and these three genera too are as yet monotypic; of Sarracenia, however, there are seven known species scattered over the eastern states of North America. The forty species of Nepenthes are mostly natives of the hotter parts of the Indian Archipelago, but a few range into Ceylon, Bengal, Cochin China, and some even occur in tropical Australia on the one hand, and in the Seychelles and Madagascar on the other. Pinguicula is abundant in the north temperate zone, and ranges down the Andes as far as Patagonia; the 250 species of Utricularia are mostly aquatic, and some are found in all save polar regions; their unimportant congeners, Genlisea and Polypompholix, occur in tropical America and south-western Australia respectively. It is remarkable that all the insectivorous plants agree in inhabiting damp heaths, bogs, marshes and similar situations where water is abundant, but where they are not brought into contact with the plenteous supply of inorganic nitrogenous food as are the roots of terrestrial plants.
INSEIN, a town of British India, in the Hanthawaddy district of Burma, 10 m. N.W. of Rangoon; pop. (1901) 5350. It is an important railway centre, containing the principal workshops of the Burma railway company, also a government engineering school, a reformatory school and the largest gaol in the province.
INSOMNIA, or deprivation of sleep (Lat. somnus), a common
and troublesome feature of most illnesses, both acute and chronic.
It may be due to pain, fever or cerebral excitement, as in delirium
tremens, or to organic changes in the brain. The treatment,
when failure to sleep occurs in connexion with a definite illness,
is part of the treatment of that illness. But there is a form of
sleeplessness not occurring during illness to which the term
“insomnia” is commonly and conveniently applied. It must
not be confounded with occasional wakefulness caused by some
minor discomfort, such as indigestion, nor with the “bad
nights” of the valetudinarian. Real insomnia consists in
the prolonged inability to obtain sleep sufficient in quantity and
quality for the maintenance of health. It is a condition of
modern urban life, and may be regarded as a malady in itself.
It is a potent factor in causing those nervous breakdowns
ascribed to “overwork.” It may occur as a sequel to some
exhausting illness, notably influenza, which affects the nervous
system long after convalescence. But it very often occurs
without any such cause. Professional and business men are the
most frequent sufferers. Insomnia is comparatively rare among
the poor, who do little or no brain work. It may be brought
on by some exceptional strain, by long-continued worry, or by
sheer overwork. The broad pathology is simple enough. It has
been demonstrated by exact observations that in sleep the
blood leaves the brain automatically. The function is rhythmical,
like all the vital functions, and the mechanism by which it is
carried out is no doubt the vaso-motor system, which controls
