284 THE INDUCTIONS OF BIOLOGY. 



of the elongated pollen-ceil applies itself to the surface of 

 the embryo-sac, and one of its nuclei having, with some 

 protoplasm, passed into the egg-cell, there becomes fused 

 with the nucleus of the egg-cell. Similarly in animals, the 

 spermatozoon passes through the limiting membrane of the 

 ovum, and a mixture takes place between the substance 

 of its nucleus and the substance of the nucleus of the 

 ovum. But the important fact which it chiefly 



concerns us to notice, is that on the union of these re- 

 productive elements there begins, either at once or on the 

 return of favourable conditions, , a new series of develop- 

 mental changes. The state of equilibrium at which each 

 had arrived is destroyed by their mutual influence, and the 

 constructive changes, which had come to a close, recommence. 

 A process of cell-multiplication is set up; and the resulting 

 cells presently begin to aggregate into the rudiment of a new 

 organism. 



Thus, passing over the variable concomitants of gamo- 

 genesis, 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 them- 

 selves distinct individuals, or are thrown off by distinct in- 

 dividuals; that these portions of organic substance, which 

 are severally distinguished by their low degree of special- 

 ization, 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 sjarvey of the facts 

 discloses certain correlations which, if not universal, are too 

 general to be without significance. 



