INTERNAL CAUSES OF VARIATION 177 



his mother or one of her male ancestors. Differences such as 

 these must arise from strictly internal causes, which seem to set 

 a natural and inevitable limit to what may be accomplished 

 through selection. Here would seem to be an irreducible mini- 

 mum in variation, arising directly through reduction. 1 



Control. In so far as variations arise through changes in 

 hereditary matter during the processes of maturation and reduc- 

 tion, they are, and must doubtless always remain, entirely 

 beyond the influence of the breeder. It is quite evident that 

 here is a degree of deviation and an element in breeding that 

 must be left to nature and subject to the laws of chance within 

 the range of characters natural to the race. That this will always 

 be a bar to absolute success is evident, but that it constitutes 

 the strongest known argument for purity of blood is, in the 

 opinion of the writer, beyond question, because the chances of 

 unfortunate deviations are reduced in proportion to the purity of 

 blood and the absence of undesirable characters. 



Variation in parthenogenetic reproduction. 2 Had Weismann's 

 original assumption been correct to the effect that sexual union 

 is the only constitutional cause of variation, then individuals 

 arising from parthenogenetic reproduction should, barring the 

 influence of surrounding conditions, be alike, because only the 



1 Endeavoring to determine the function of the cytoplasm and nucleus, Boveri 

 removed the nuclei from the eggs of sea urchins and afterward admitted sperma- 

 tozoa to these enucleated ova. Development followed in a few cases, but the 

 nuclei were smaller than in larvae normally fertilized, contained but half the num- 

 ber of chromosomes, and the resulting larvae possessed the "pure parental characters." 



It is not supposable that anything like this occurs in nature, and yet it raises 

 the question whether, after reduction, every remaining element of the nucleus of 

 both parents always plays its part in development. Should it fail to do so for any 

 reason, herein would lie a sufficient cause for the occasional remarkable resem- 

 blance of offspring to one and not the other parent. 



2 While in all higher animals and plants a union of a male with the female cells 

 is necessary to each fertilization and to the production of young, it is by no means 

 true among other organisms, especially in rotifers, crustaceans, and insects with 

 which "parthenogenesis has become a fixed physiological habit," through which 

 the unfertilized female cell develops a perfect individual. 



It is now well known that the queen of the honeybee, if prevented from mat- 

 ing, will yet lay eggs capable of development, but they will all be drones (males). 

 After mating she can lay either fertilized or unfertilized eggs, the fertilized devel- 

 oping into workers (undeveloped females), or, if properly fed, into queens, 

 the unfertilized into drones as before. After the male element is exhausted (she 



