consists of nine segments, some of which are often fused together. It contains the various internal organs, of which the most important are those of Digestion and Reproduction. The _Digestive System_ (Plate I., fig. 10) consists of the following organs: A, the _oesophagus_, or throat; C, the _sucking stomach_; D, the _ventriculus_ or stomach; E, the _small intestine_; G, the _cæcum_; H, the _colon_; K, the _biliary vessels_; N, the _salivary vessels_. The function of the _sucking stomach_ is to exhaust the air in the throat and proboscis, and thus to cause the ascent of the fluids into the stomach when the insect is feeding. III.--ORIGIN OF SPECIES. The theory of the origin of species as propounded by Darwin may be thus very briefly summarised:-- {xiv}VARIATION.--No two organisms are exactly alike; there is always some variation from the parent form, in some cases very slight, in others considerable. (For examples of variation see Plate VII., figs. 1 to 9, varieties of _Hydriomena deltoidata_; Plate VIII., figs. 42 to 47, varieties of _Epirranthis alectoraria_; Plate IX., figs. 6 to 14, varieties of _Selidosema productata_; Plate X., figs. 13 to 23, varieties of _Azelina gallaria_; Plate X., figs. 39 to 47, varieties of _Declana floccosa_.) INHERITANCE.--Many of these variations are inherited--a fact demonstrated by our domestic plants and animals, where man has selected and bred from varieties suitable for his purposes, and has thus produced races in which the variation is permanent. Many of the races of domestic animals differ as much from one another as do some distinct species of wild animals. STRUGGLE FOR EXISTENCE.--All animals and plants produce far more offspring than can possibly survive, thus giving rise to the struggle for existence. For example: The average number of eggs laid by a Lepidopterous insect is certainly over 100, and in many species this number is greatly exceeded. Assuming each female to lay 100 eggs, the progeny from a single pair would amount, after six generations, to over six million individuals. NATURAL SELECTION, or the SURVIVAL OF THE FITTEST.--In the struggle for existence which necessarily results from such a great increase of individuals, those variations which favoured the possessors would be preserved, whilst those which did not, would be gradually exterminated. This principle of the preservation of the favourable varieties in the struggle for life is called Natural Selection, or the Survival of the Fittest.