Heredity, like other biologic factors, starts with the cell. As elsewhere pointed out, reproduction is first unicellular in type and involves an expenditure of nutritive force antagonistic to the growth of the cell. As Geddes remarks,[66] no one can dispute that the nutritive, vegetative, or self-regarding processes within the plant or animal are as opposed to the reproductive, multiplying, species-regarding processes, as income to expenditure or as building up to breaking down. But within the ordinary nutritive or vegetative functions of the body there is necessarily a continuous antithesis between two tissue-changing sets of processes, constructive and destructive metabolism. The contrast between these two processes is seen throughout nature, whether in the alternating phases of cell-life or in the great antithesis of growth and reproduction. The starfish, deprived of an arm, replaces this by a fresh growth; crabs can renew the great claws which they have lost in fighting; even as high up as the lizard the loss of a leg or a tail can be made good. In a great variety of cases a kind of physiological forgiveness is shown in the reparation of even serious injuries. Now this "regeneration," as it is called, is a process of reproduction. By continuous growth the cells of a persistent stump are able to reproduce the entire number. A sponge, a hydra or a sea-anemone may be cut into pieces with the result that each fragment grows into a new organism. The same is done with many plants; and though the division is artificial the result shows how very far from unique is the process spoken of as reproduction, which is but more or less discontinuous growth. This is well shown in the evolution onward insensibly from cases of continuous budding, as in sponge or rosebush, to discontinuous budding in hydra zoophyte and tiger-lily, where the offspring vegetatively produced are sooner or later set free. The enormous expenditure of force required for unicellular reproduction is lessened by conjugation with another cell through satisfaction of cell hunger; and this, by making two cells do the work of one, lessens the amount of nutritive force expended by each. Evolution in fertilisation has the following steps:-- I. Formation of plasmodia. II. Multiple conjugation. III. Conjugation of the two similar cells. IV. Union of incipiently dimorphic (different) cells. V. Fertilisation of differentiated sex elements.