CHAPTER 7 



SPECULATIONS CONCERNING THE EVOLUTION 

 OF THE SEX-DETERMINING MECHANISM 



It is possible that at a certain stage in the history of the 

 earth the conditions essential for the appearance of life pre- 

 sented themselves, never afterwards to be repeated, peculiar 

 in respect of temperature, pressure, of the composition of 

 the waters and of the gases in the atmosphere above the 

 waters. It is possible that the conditions at that time exist- 

 ing led to the appearance of living matter as inevitably as 

 earlier and different sets of conditions had led to the form- 

 ation of the seas and the rocks. 



The first living or half-living things which appeared in 

 the waters were possibly large molecules synthetized under 

 the influence of the sun's radiation and capable of repro- 

 duction only in this particularly favourable medium. 



A review of living things now^ known to us permits us to 

 assume that the enzyme, the virus and the bacteriophage 

 are perhaps milestones along the beginning of the road that 

 life has passed onwards and upwards toward its inevitable 

 destiny. If they cannot be seen they can be recognized 

 and counted by the effects they produce. Muller (1929) has 

 suggested that the bacteriophage is a gene.^ It may well be 

 that life remained in this stage of its development for many 

 millions of years before a suitable assemblage of similar 

 units was brought together in the first cell. There must have 

 been innumerable failures, but the first successful cell which 

 consisted of numerous half-living chemical molecules 

 suspended in water and enclosed in an oily film found plenty 

 of food and an immense advantage over its competitors. 

 From this original simple colloidal complex to the first and 

 simplest unicellular organism known to the biologist is a 



^ For a fuller account of bacteriophage and of its nature the 

 reader is referred to Dr. Gardner's excellent monograph on 

 Microbes and Ultramicrobes in this series. 



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