The vital _principle_, represented in the _sperm_-cell by a spermatozoön, must be imparted to a _germ_-cell in order to effect impregnation. After touching each other, separate them immediately, and observe the result. If, with the aid of a powerful lens, we directly examine the spermatozoön, it will be perceived that, for a short time, it preserves its dimensions and retains all its material aspects. But it does not long withstand the siege of decay, and, having fulfilled its destiny, loses its organic characteristics, and begins to shrink. If we examine the fertilized germ, we discover unusual activity, the result of impregnation. Organic processes succeed one another with wonderful regularity, as if wrought out by inexplicable intelligence. Here begin the functions which constitute human physiology. Generation requires that a spermatozoön be brought into actual contact with a germ that fecundation may follow. If a spermatic cell, or spermatozoön, together with several unimpregnated ova, no matter how near to one another, if not actually touching, be placed on the concave surface of a watch-crystal, and covered with another crystal, keeping them warm, and even though the vapor of the ova envelops it, no impregnation will occur. Place the spermatozoön in contact with an ovum, and impregnation is instantly and perfectly accomplished. Should this vitalizing power be termed nerve-force, electricity, heat, or motion? It is known that these forces may be metamorphosed; for instance, nervous force may be converted into electricity, electricity into heat, and heat into motion, thus illustrating their affiliation and capability of transformation. But nothing is explained respecting the real nature of the vital principle, if we assert its identity with any of these forces; for who can reveal the true nature of any of these, or even of matter? ALTERNATE GENERATION. In several insect families, the species is not wholly represented in the adult individuals of both sexes, or in their development, but, to complete this series, supplementary individuals, as it were, of one or of several preceding generations, are required. The son may not resemble the father, but the grandfather, and in some instances, the likeness re-appears only in latter generations. Agassiz states: "Alternate generation was first observed among the Salpae. These are marine mollusks, without shells, belonging to the family Tunicata. They are distinguished by the curious peculiarity of being united together in considerable numbers so as to form long chains, which float in the sea, the mouth(_m_) however being free in each. [Illustration: Fig. 2. ] [Illustration: Fig. 3. ] "Fig. 2. The individuals thus joined in floating colonies produce eggs; but in each animal there is generally but one egg formed, which is developed in the body of the parent, and from which is hatched a little mollusk.