188 THE THEORY OF THE GENE 



them, lies in the explanation they offer of a peculiar and 

 interesting genetic situation arising from the occasional 

 erratic behavior of the mechanism that is involved in the 

 processes of cell division and maturation. Unstable forms 

 are produced, that, in so far as they maintain themselves, 

 do so by remaining unstable, i.e., with an extra chromo- 

 some. In this respect they differ obviously from normal 



central 



lata / m V \ scintillans 



V 



>allescens \ m m cana 



liquida \ ^ / spathulata 



Fig. 107. 

 Diagram illustrating de Vries' idea of the relation between the 

 seven chromosomes of O. Lamarckiana and types of trisomic mu- 

 tants. 



types and species. Furthermore, most of the evidence 

 indicates that these heteroploids are not so viable as the 

 balanced types from which they arise, hence would rarely 

 be able to replace them or act as substitutes in a different 

 environment. 



Nevertheless, the occurrence of heteroploidy must be 

 regarded as a significant genetic event whose explanation 

 promises to clear up many situations that would be very 

 puzzling without the information which a study of their 

 chromosomes has revealed. 



