232 AN INTRODUCTION TO MODERN GENETICS 



Altenburg^ in terms of the most economical way of performing the 

 "work" necessary for reproduction. 



The considerations given above suggest why evolution should have 

 produced sexuality, but they are not adequate to explain exactly how 

 sexual differentiation is caused. For that we require a physiological 

 theory. Biitschli and later Schaudiim put forward what is known as the 

 SexuaUty Theory, which has been thus expressed by its main modern 

 exponent, Hartmann i^ 



"Every Protist or sex-cell, in fact every cell at all, is hermaphrodite 

 or bisexual and has the full germs or potencies of the male and 

 female sex. By the preponderant development of one or the other 

 potency, a cell becomes male or female in comparison with other cells 

 in which the opposite potency has been realized. In this way the cells 

 acquire a male or female tendency." 



Hartmann has developed this point of view into a comprehensive 

 theory of sexuality. Using Correns's symbolism, he postulates for all 

 cells an underlying alternative reaction system AG, where A represents 

 potentialities for male development, G those for female development. 

 These he represents twice in a diploid cell AAGG and once in a haploid 

 cell AG, but he considers that they are of the same nature throughout, 

 and that sexuality, to some extent at least, is the same thing in the 

 gamo- and zygo-phases. Acting specifically on these underlying com- 

 plexes, Hartmann postulates "realizers" a and y, which exist in allelo- 

 morphs of different strengths. 



2. The Alternative Reaction Systems 



This theory can be taken as the starting point of our discussion. The 

 first point which needs elucidation is the nature of the alternative 

 reaction systems A and G. We cannot suppose that the first living 

 matter was endowed with such dual potencies. Moreover, there are 

 forms of life at the present day in which a bipolar sexual differentiation 

 is apparently absent; for instance, the so-called isogamous Algae in 

 which the gametes all seem to be equal. It is difficult to distinguish two 

 sexes in organisms Hke Spirogyra or Ciliates, and impossible, without 

 elaborate subsidiary hypotheses, in Fungi with multipolar sexuality. 

 Even in those organisms which show two alternative sexes, it is difficult 

 to say how the so-called male in one group resembles the male in 

 another. Hartmann avoids the difficulty by saying that the fundamental 

 ^ Altenburg 1934a. ^ Hartmann 1931. 



