CHAPTER II 

 MENDELISM AND THE HEREDITY OF SEX 



We know that new individuals are developed from 

 single cells which have either been formed by the 

 union of two cells or which develop without such 

 union, and that these reproductive cells are separated 

 from pre-existing organisms : the gametes or 

 gonocytes are separated from the parents and 

 develop into the offspring. The zygote has the 

 power of developing particular structures and 

 characters in the complicated organisation of the 

 adult, and we recognise that these characters are de- 

 termined by the properties and constitution of the 

 zygote ; that is to say, of one or both of the gametes 

 which unite to form the zygote. The distinction 

 between peculiarities or ' characters,' determined 

 in the ovum before development, and modifications 

 due to influences acting on the individual during 

 its development or life, is often obvious enough. 

 A child's health, size, mode of speech, and behaviour 

 may be greatly influenced by feeding, training, and 

 education, but the colour of his or her eyes and hair 

 were determined before birth. A human individual 

 has, we know, a number of congenital or innate 

 characters, by which we mean characters which 

 arise from the constitution of the individual at the 

 time of birth, and not from influences acting on him 

 or her after birth. We have to remember, however, 



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