2 SEX-DETERMINATION 



known concerning these matters. Up to the beginning of the 

 present century each of them in its turn was destroyed when 

 it proved to be incapable of accommodating some new 

 observation. In retrospect it is easy to understand how it 

 came about that a theory derived from and based upon the 

 experience of an obstetrician, for example, could not be 

 stretched to include the outcome of the experience of a 

 breeder of habitually polytocous livestock. 



Then, as the studies of the zoologist and of the botanist 

 widened to include an ever-expanding number of species, it 

 is understandable how it happened that a theory elaborated 

 by a zoologist proved to be of no value whatsoever to a 

 botanist who had encountered in his material phenomena 

 strongly resembling those of sexuality in the animal. That 

 for which men continually sought was a theory that could 

 accommodate all that was known about the phenomenon of 

 sexuality wherever it appeared, and as this knowledge ex- 

 panded the difficulties of constructing a satisfactory theory 

 of the causation of sex multiplied. 



Each of these theories of sex-determination has to be 

 examined against the background of the total biological 

 knowledge that was in man's possession at the time when 

 the theory was promulgated. If it accommodated all the 

 observations thus far made, and if it was a reasonable, 

 intelligent groping after understanding, then in its day it was 

 a good and useful theory. That it is now unwarrantable in 

 the light of our vastly increased knowledge of matters 

 biological in no way robs the theory of its merit. 



By the beginning of this century our knowledge of the 

 cell, of gametogenesis and of fertilization had become 

 greatly expanded, and in the earliest years of the century, 

 as an outcome of the confirmation of the Mendelian theory 

 of organic inheritance, much attention became focused upon 

 the mode of transmission of inherited characters and the 

 search began for the actual mechanism of segregation that 

 was postulated by the Mendelian theory. Thus it was that 

 the sciences of genetics and of cytology entered into a phase 

 of intensely active development. 



The facts to be accommodated by a theory of sex- 



