Another group shows that two forms of Crows which appear quite distinct, and, judged by their external characters, might be regarded as different species, may in a state of nature unite, and produce hybrid offspring. In the same case is exhibited a series of Goldfinches to show a complete gradation between birds of different colouring, which have been regarded as different species. Both these examples may by some naturalists be considered instances, not of the crossing of distinct species, but of “dimorphism,” or the occurrence of a single species in nature in two different phases. From whatever point of view they may be regarded, they illustrate the difficulty of defining and limiting the meaning of the term “species,” of such constant use in natural history. [Sidenote: Mosquitoes and Tsetse-Flies.] In the middle line of the hall are placed cases containing greatly enlarged models of certain Insects concerned in the spread of disease, such as Mosquitoes or Gnats (figs. 3 and 4), a House-Fly, Tsetse-Fly (fig. 2), and Plague-Flea; also still more enlarged gelatine models of mammalian blood-corpuscles, showing the parasites by which they are infested in the diseases respectively communicated by means of Mosquitoes and Tsetses. Models of the parasites themselves are also shown (figs. 5 and 6). Malaria, or ague, is a disease caused by extremely minute parasites which live in the red corpuscles of the blood. Formerly malaria was believed to be contracted by merely breathing the air of marshy districts, but it is now proved that the parasites are transmitted from man to man by the “bite,” or rather “stab,” of a Mosquito or Gnat. The Common Mosquito or Stabbing Gnat (_Culex pipiens_), fig. 3, does not transmit the malaria-parasite; the Spot-winged Mosquitoes (fig. 4) of the genus _Anopheles_, abundant in England and nearly all parts of the world, being the carriers. The parasite multiplies not only in the human blood, but in the stomach and tissues of the Gnat--as shown in the models (fig. 5). Tsetse-Flies are African blood-sucking Flies, with the mouth-parts adapted for piercing the skins of wild and domesticated mammals, human beings, and even crocodiles. The blood of some of the larger African animals is sometimes infected with a microscopic parasite (fig. 6), which, if sucked up by Tsetse-Flies when feeding, and subsequently introduced by them into the veins of domesticated animals such as horses and cattle, produces the fatal _nagana_, or Tsetse-Fly disease. Sleeping-sickness in man is caused in a similar manner, and is conveyed from infected to healthy individuals by two kinds of Tsetse-Fly, including that of which an enlarged model, 28 times (linear) natural size, is exhibited. The parasites and the red blood-corpuscles are enlarged 6,000 diameters. [Illustration: FIG. 3.--(_a_) THE COMMON GNAT (_Culex pipiens_). Enlarged. FIG. 4.--(_b_) THE SPOT-WINGED MOSQUITO (_Anopheles maculipennis_). Enlarged.]