GENESIS, HEREDITY, AND VARIATION. 347 



fertilization will be effective in various degrees. Hence it may 

 happen that among offspring of nearly-related parents, there 

 may be some in which the want of vigour is not marked, and 

 others in which there is decided want of vigour. So that we 

 are alike shown why in-and-in breeding tends to diminish both 

 fertility and vigour : and why the effect cannot be a uniform 

 effect, but only an average effect. 



96. While, if the foregoing arguments are valid, gamo- 

 genesis has for its main result the initiation of a new develop- 

 ment by the overthrow of that approximate equilibrium 

 arrived at among the molecules of the parent-organisms, a fur- 

 ther result appears to be subserved by it. Those inferior or- 

 ganisms which habitually multiply by agamogenesis, have con- 

 ditions of life that are simple and uniform; while those 

 organisms which have highly-complex and variable conditions 

 of life, habitually multiply by gamogenesis. Now if a species 

 has complex and variable conditions of life, its members must 

 be severally exposed to sets of conditions that are slightly 

 different : the aggregates of incident forces cannot be alike 

 for all the scattered individuals. Hence, as functional 

 deviation must ever be inducing structural deviation, each 

 individual throughout the area occupied tends to become 

 fitted for the particular habits which its particular conditions 

 necessitate; and in so far, wnfitted for the average habits 

 proper to the species. But these undue specializations are 

 continually checked by gamogenesis. As Mr. Darwin remarks, 

 " intercrossing plays a very important part in nature in 

 keeping the individuals of the same species, or of the variety, 

 true and uniform in character : " the idiosyncratic divergences 

 obliterate one another. Gamogenesis, then, is a means of 

 turning to positive advantage the individual differentiations 

 which, in its absence, would result in positive disadvantage. 

 Were it not that individuals are ever being made unlike one 

 another by their unlike conditions, there would not arise in 

 them those contrasts of molecular constitution, which we have 



