CHAPTER X. 



GENESIS, HEREDITY, AND VARIATION 1 ". 



92. A QUESTION raised, and hypothetical!}- answered, in 

 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 which the facts suggest. 



After contemplating the several methods by which the 

 multiplication of organisms is carried on after ranging them 

 under the two heads of Homogenesis, in which the succes- 

 sive generations are similarly produced, and Heterogenesis, 

 in which they are dissimilarly produced after observing that 

 Homogenesis is nearly 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 

 answers to these questions, 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? 

 336 



