EFFECT OF BI-PARENTAL REPRODUCTION 129 



grandparent, but after an ancestor thousands of generations 

 removed, an ancestor probably far more like the Somali than 

 any of the Burchell zebras." : This is only one among many 

 examples which all tend to show that the effect of bi-parental 

 reproduction is generally markedly the reverse of the pro- 

 duction of new variations. Usually any great variations 

 would interfere with the adaptation of the organism to its 

 environment. Natural selection, therefore, must tend to 

 eliminate great variations as a rule. Bi-parental repro- 

 duction tends to plane away all those variations which are 

 not the subject of selection, or which are not present in both 

 parents. The tendency to large variations being itself a 

 variation for it is not probable that all individuals will 

 tend to vary to the same extent this character must be 

 just as much the subject of selection as are other characters, 

 and as the environment does not generally change rapidly } 

 natural selection usually chooses those individuals which 

 exhibit only small variations. It would appear then that 

 " under conditions of Natural Selection, bi-parental repro- 

 duction insures that all evolution shall be on lines of small 

 variations, not on lines of great abnormalities." 2 



The conclusion arrived at is, that bi-parental reproduction 

 " is an exceedingly effective device to assist and direct the 

 retrogression which is the necessary result of development 

 by recapitulation ; for unlike the latter, it is selective in 

 action. It eliminates as a rule only useless variations. . . . 

 Bi-parental reproduction, indeed, is only another name for 

 bi-parental selection." 3 



Other writers have arrived at the conclusion that bi- 

 parental reproduction does not cause variations. " Variability 

 is not a product of bi-parental inheritance. . . . Whatever 

 be the physiological function of sex in evolution, it is not 

 the production of greater variability." 4 



1 Penicuik Experiment!, p. xii. 



2 Archdall Reid, op. cit. 



3 Ibid. 



4 Pearson, Karl, Grammar of Science, 1900. 



