350 The Unity of the Organism 



caused by the extra chromosome of the sperm cell. Two 

 distinct questions suggest themselves to the critically 

 minded: assuming it proved that spermatozoa having extra 

 chromosomes do induce eggs to develop into males in some 

 animals, how generally is this true for sexually propagating 

 organisms? And assuming it true either universally or only 

 in a few animals, what is the real meaning of the statement 

 that the accessory element of the sperm is the "bearer of 

 those qualities which pertain to the male organism"? 



The answer to the first question alone concerns us now, 

 though the answer to the second is far more fundamental 

 and upon it depends in large measure the significance of 

 whatever answer may be forthcoming to the first. 



The arguments by which McClung supported his hypothe- 

 sis were rather general and indirect, and it is possible to 

 state in a single sentence the main outcome of later research 

 relative to it. A connection between sex and particular 

 chromosomes has been definitely proved for a large number 

 of animals ; but the particular connection supposed by Mc- 

 Clung, namely, that the accessory chromosome of the sperm 

 produces a male, has not been proved. In 1911 Wilson, epit- 

 omizing the results of his own researches and those of others 

 using terms necessitated by discoveries since 1902, said, 

 "The observed relations of the X- and Y-chromosomes to 

 sex are not theories, but facts." ^^ The evidence seems un- 

 doubtedly to justify this statement; so information as to 

 what the X- and Y-chromosomes are will furnish informa- 

 tion of the dependence of sex upon chromosomes. 



Discoveries were made soon after the enunciation of Mc- 

 Clung's hypothesis that seemed almost certainly to connect 

 the extra chromosome of the sperm not with the production 

 of a male, but of a female. "The decisive evidence," writes 

 Wilson, "in regard to this question was first produced by 

 independent investigations upon Hcnii})tcra and Coleoptcra 

 by Miss Stevens and myself in 1905-1906.'* ^^ This evidence 



