150 SUMMARY AND CONCLUSION. 



of the character thus discriminated against. Balanced selection is 

 usually secured by selecting individuals of the average form, and 

 tends to produce increasing stability. 



3. Cumulative Effects through the Cooperation of Different Principles. 



Two or more of the factors mentioned in this volume may co- 

 operate in rendering a type more stable, or in rendering its previously 

 attained characters more intense, or in diminishing its present char- 

 acteristics while others are brought into prominence. If the organ- 

 isms under consideration form but one intergenerant, any transforma- 

 tion thus produced will be monotypic ; but if through the cooperation 

 of isolation they form several intergenerants, any subsequent trans- 

 formation will result in polytypic evolution. 



Again, if each factor working by itself would tend to produce the 

 same result, the united influence of several factors working together 

 will be much more decisive than that of but one of them working alone. 



4. Cumulative Effects through the Operation o) the Same Principle in 

 Successive Generations. 



Once more it should be noted that the effect of unbalanced selection 

 when continued through many successive generations is vastly 

 greater than when lasting but for one generation. Indeed, on reflec- 

 tion it becomes apparent that the great difference between selective 

 survival and non-selective survival is that the former is continuous 

 from generation to generation, while the latter is accidental, and, 

 therefore, not continuous. Moreover, in non-selective survival the 

 effects of survival in any one generation are liable to be in a measure 

 neutralized by the effects of survival in succeeding generations. Dis- 

 criminate isolation is more effective than indiscriminate isolation 

 because it is more effective in bringing together in one group a consid- 

 erable number of individuals that belong to the same class ; that is, 

 that are either of average character, or above the average, or below the 

 average. Indiscriminate isolation is less likely to bring together a 

 special type and to repeat the process through many generations, 

 and is, therefore, usually less effective than discriminate isolation in 

 producing transformation. 



The probability that a cumulative result will be reached through 

 the effects of indiscriminate isolation, dividing the whole species into 

 two large and nearly equal groups, without the cooperation of selec- 

 tion or without the continuous and cumulative effects of suetude (i. e., 

 of use or disuse), is very small; but we must remember that when 

 isolation has become effective in shutting out all individuals of other 

 groups, divergent selection, divergent suetude, and different forms 



