CHAPTER X. 



GENESIS, HEREDITY, AND VARIATION. 



§ 92. A QUESTION raised, and hypotlietically answered, in 

 g§ 78 and 79, was there postponed until we had dealt with the 

 topics of Heredity and Variation. Let us now resume the 

 consideration of this question, in connexion with sundry 

 others w^hich the facts suggest. 



After contemplating the several methods by which the 

 multiplication of organisms is carried on — after ranging 

 ihem under the two heads of Homogenesis, in which the suc- 

 cessive generations are similarly produced, and Heterogenesis, 

 in which they are dissimilarly produced — after observing 

 that Homogenesis is always sexual genesis, while Heteroge- 

 nesis is asexual genesis with occasionally-recurring sexual 

 genesis ; we came to the questions — why is it that some or- 

 ganisms multiply in the one way, and some in the other ? 

 and why is it that where agamogenesis prevails, it is usually, 

 from time to time, interrupted by gamogenesis ? In seeking 

 an answer to this question, we inquired whether there are, 

 common to both Homogenesis and Heterogenesis, any condi- 

 tions under which alone sperm-cells and germ-cells arise and 

 are united, for the production of new organisms ; and we 

 reached the conclusion that, in all cases, they arise only 

 when there is an approach to equilibrium between the forces 

 which produce growth and the forces which oppose growth. 

 This answer to the question — when does gamogenesis recur 'i 



