52 SEX-DETERMINATION 



Step as vast as that which separates the latter and man. We 

 know nothing of this grand procession; we can but con- 

 jecture that it was punctuated by the development of various 

 precise mechanisms. From what we know of the gene today 

 we can surmise that the earliest genes consisted of molecules 

 capable of determining the formation of similar particles 

 and also of dissimilar particles. We may assume that repro- 

 duction by simple division attended upon growth and that 

 every few hours a new generation of these units was sub- 

 jected to the appraisal of the selecting factors in the changing 

 environment. At some stage in this eventful history there 

 must have come a time when the gene, dividing, became 

 two which did not separate but which remained together so 

 that the beginnings of a gene company would be evolved. 

 Since we know that genes in such a company can mutate 

 independently, it follows that through the increasing com- 

 plexity thus resulting advantages possessing a survival value 

 would be conferred upon the individual. 



The benefits of gene association must then have been 

 made more permanent by the development of mitosis, 

 which development would take the form of the establish- 

 ment of a mechanism which ensured the synchronous 

 division of all the genes. Thus the nucleus would be evolved 

 and within it the gene associations would become linear and 

 the chromosomes would be formed, and, for their exact 

 division, the centrosome, spindle and the spindle attach- 

 ment would be evolved. Fragmentation, with the develop- 

 ment of new spindle attachments, translocation, together 

 with frequent gene mutation, would slowly, surely, build 

 up permanent gene associations which would yield different 

 types of genie balances to be appraised and selected. At this 

 stage, sexual reproduction would appear, possibly as a 

 result of a gene mutation which made the fusion of two 

 individuals inevitable. Following upon this, meiosis — a 

 modification of mitosis — must have appeared, bringing with 

 it two exceedingly great advantages; the maintenance of 

 constancy in chromosome number and the provision of the 

 conditions of crossing over with consequent recombination 

 of genes and reconstruction within the chromosome. It may 



