284 THE INDUCTIONS OF BIOLOGY. 



have resulted, either an undue likeness, or a due tmlike- 

 ness, between the married cousins. These several causes, 

 conspiring and conflicting in endless ways and degrees, will 

 work multiform effects. Moreover, differences of segrega- 

 tion will make the reproductive centres produced by the 

 same nearly-related organisms, vary considerably in their 

 amounts of unlikeness ; and therefore, supposing their amounts 

 of unlikeness great enough to cause fertilization, this fertiliza- 

 tion 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 end, the initiation of a new develop- 

 ment by the overthrow of that approximate equilibrium arrived 

 at among the molecules of the parent-organisms ; a further end 

 appears to be subserved by it. Those inferior organisms 

 which habitually multiply by agamogenesis, have conditions 

 of life that are simple and uniform ; while those organisms 

 that have highly-complex and variable conditions of life, 

 habitually multiply by gamogenesis. ISTow 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, w^fitted 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 



