204 THE EVOLUTION THEORY 



Quetelet and Galton have attempted to show that intercrossing leads 

 to a mean which then remains constant. Haycraft supposes that 

 a species can only remain constant if its individuals are being con- 

 tinually intercrossed, and that otherwise they would diverge and take 

 difi'erent forms, because the ' protoplasm ' has within itself the tendency 

 to continual variation. The transformation of species is effected by 

 means of this variation tendency, and the persistency and constancy 

 of species which are already adapted to the conditions of their life 

 are secured by the constant intercrossing of the individuals, and the 

 consequent neutralization of individual peculiarities. 



Although the cases already mentioned in which great constancy 

 of species is associated with pui-ely parthenogenetic reproduction do not 

 tell in favour of the accuracy of the view just stated, yet the funda- 

 mental idea, that amphigony is an essential factor in the main- 

 tenance and even in the evolution of species, is undoubtedly sound. 

 We should certainly find neither genera nor species in Nature if 

 amphigony did not exist ; l)ut we cannot simply suppose that 

 amphigony and variation are, so to speak, antipodal forces, the 

 former of which secures the constancy of the species, the latter its 

 transformation. In my opinion, at all events, there is no such thing 

 as a ' tendency ' of the protoplasm to vary, although there is a constant 

 fluctuation of the characters — dependent on the imperfect equality of 

 the external influences, especially of nutrition. This certainly results, 

 as far as it takes place within the germ-plasm, in a continual upward 

 and downward variation of the hereditary tendencies, and it would 

 lead to increasing dissimilarity of the individuals were it not that 

 amphigony is continually equalizing the differences by a constantly 

 repeated mingling of individuals. Quetelet and Galton have shown 

 7 that the tendency of this mingling is towards the establishment of 

 a mean ; the characters of Man, such as bodily size, fluctuate about 

 a mean, which at the same time shows the maximum of frequencjT- ; 

 and the frequency curve of the various bodily sizes assumes a perfectly 

 symmetrical form, so that the average size is the most frequent, and 

 deviations from it upwards or downwards occur more rarely in jDro- 

 portion to the amount of deviation, the largest ami the smallest sizes 

 occurring least frequently. 



Thus an equalizing of variations by means of amphimixis really 

 exists, and the question we have to ask is. How does it come about ? 

 The case is assuredly not the same as that in which equal quantities 

 of red and white wine are mixed to make a so-called ' Schiller.' This 

 is proved even hy the fact that the inixture ma3^ turn out quite 

 different even when the wines — the two parents — are alike : for the 



