La vie animale et l'intelligence . ay tous les animaux vieillissent et meurent ; mais nous pouvons encore dire que tous les animaux, à l'exception possible de certains d'thelowest et la plus simple, la pièce, après une plus longue ou une shortertime, une diminution de l'énergie vitale qui, tôt ou laterends dans la mort. 10. Animaux reproduire leur genre. Nous venons de voir la nature de la vie animale. i 3 la nature de la reproduction dans l'amibe unicellulaire simple.La reproduction des cellules de l'organisme, complexmulticellular au cours de sa croissance naturelle ou rendre bonne la perte inévitable à la suite de l'usure andtear de vie

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Animal life and intelligence . ay thatall animals grow old and die; but we may still say thatall animals, with the possible exception of some of thelowest and simplest, exhibit, after a longer or a shortertime, a waning of the vital energies which sooner or laterends in death. 10. Animals reproduce their kind. We have just seen The Nature of Animal Life. i 3 the nature of reproduction in the simple unicellular amoeba.The reproduction of the constituent cells in the complexmulticellular organism, during its natural growth or tomake good the inevitable loss consequent on the wear andtear of life, is of the same character. When we come to the higher organisms, reproductionis effected by the separation of special cells called egg-cells, or ova, from a special organ called the ovary; and these, in a great number of cases, will not develop into a neworganism unless they be fertilized by the union with themin each case of another cell—the sperm-cell—produced by adifferent individual. The separate parents are called male. h Fig. 5.—Egg-cell and sperm-cell.a, ovum or egg; b, spermatozoon or sperm. and female, and reproduction of this kind is said to besexual. The wonderful thing about this process is the power ofthe fertilized ovum, produced by the union of two minutecells from different parents, to develop into the likeness ofthese parents. This likeness, however, though it extendsto minute particulars, is not absolute. The offspring is notexactly like either parent, nor does it present a precisemean between the characters of the two parents. Thereis always some amount of individual variability, the effectsof which, as we shall hereafter see, are of wide importance.We are wont to say that these phenomena, the transmis-sion of parental characteristics, together with a margin of 14 Animal Life and Intelligence. difference, are due to heredity with variation. But thismerely names the facts. How the special reproductivecells have acquired the secret of developing along spec

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