Thus, passing over the variable concomitants of gamogenesis, and confining our attention to what is constant in it, we see:--that there is habitually, if not universally, a fusion of two portions of organic substance which are either themselves distinct individuals, or are thrown off by distinct individuals; that these portions of organic substance, which are severally distinguished by their low degree of specialization, have arrived at states of structural quiescence or equilibrium; that if they are not united this equilibrium ends in dissolution; but that by the mixture of them this equilibrium is destroyed and a new evolution initiated. § 78. What are the conditions under which Genesis takes place? How does it happen that some organisms multiply by homogenesis and others by heterogenesis? Why is it that where agamogenesis prevails it is usually from time to time interrupted by gamogenesis? A survey of the facts discloses certain correlations which, if not universal, are too general to be without significance. Where multiplication is carried on by heterogenesis we find, in numerous cases, that agamogenesis continues as long as the forces which result in growth are greatly in excess of the antagonist forces. Conversely, we find that the recurrence of gamogenesis takes place when the conditions are no longer so favourable to growth. In like manner where there is homogenetic multiplication, new individuals are usually not formed while the preceding individuals are still rapidly growing--that is, while the forces producing growth exceed the opposing forces to a great extent; but the formation of new individuals begins when nutrition is nearly equalled by expenditure. A few out of the many facts which seem to warrant these inductions must suffice.