112 MAN, THE ANIMAL 



They both contribute the raw materials out of 

 which is constructed his intelligent reactions. 



The modern students of heredity have thus fixed 

 their attention upon structural and physiological 

 characters and not upon their first origin. The 

 origin of a hand, foot, ear, hair, feathers, brain 

 or a four-parted heart are usually considered 

 under the broader term of evolution. 



Are there definite physical bodies which carry 

 these numerous parental characters to the off- 

 spring? This important question has been an- 

 swered in the chapter on The Law of Biogenesis. 

 There it was shown that the chromatin during 

 reproduction took on a specific form and was 

 aggregated into definite masses that became 

 known as chromosomes. (See page 76.) The 

 chromosomes were shown to be equal in number 

 in the male and female parents for all animals that 

 reproduce sexually. To this generalization there 

 is one exception upon which much emphasis is 

 placed. For about this single fact is centered the 

 modern explanation of sex. It has been observed 

 by many critical students of cytoloty that during 

 maturation an extra chromosome can be distin- 

 guished In part of the sperms. This extra 

 chromosome Is known as the sex-chromosome and 

 Is believed to contain the substance that produces 

 femaleness. In so far as our technical studies 



