HEREDITY AND SEX 119 



sex only, and are believed to be united within one chromosome 

 with the sex determiner. Examples of such characters are the 

 tortoiseshell colour of female cats, which is transmitted by the 

 yellow male, and various diseases in man. Morgan has shown 

 that sex-linked inheritance is not uncommon in insects as well 

 as in birds. The breaking of such linkage, when it occurs, is 

 believed to take place in synapsis (see p. 37) ; in this process, 

 which is known as " crossing over," the genes are supposed to 

 pass from one paired chromosome to the other at the time when 

 these are drawn out into slender threads closely twisted about 

 one another so that they are hard to distinguish. 



The Inheritance of Acquired Characters 



It will be seen that the Mendelian tlieory, while explaining 

 the inheritance of congenital characters and the mechanism 

 whereby these are distributed among the offspring, takes no 

 account of spontaneous variation. Still less does it in any way 

 suggest a solution of the problem which Darwin set out to solve, 

 viz., the origin of new species. For under Darwin's theory of 

 the formation of new types by natural selection or the survival 

 of the fittest, the fact of variation was taken for granted ; this 

 was the material upon which natural selection worked, and no 

 attempt was made to explain how new variations came into 

 existence. It is clear, however, that unless we reject altogether 

 the theory of descent by modification, we must postulate the 

 occurrence at some stage in evolution — and possibly a very remote 

 stage — of the permanent acquirement of new characters, and it is 

 difficult to resist the conclusion that these were brought into 

 being through the action of the environment. This is another 

 way of saying that characters acquired in the life history of the 

 individual through its reaction to its surroundings, must have 

 been transmitted to subsequent generations of offspring. 



Of the inheritance of acquired characters in its crude form 

 there is little or no evidence. As is well known, circumcision 

 has been practised by certain races of mankind for a great number 

 of generations, and yet there is no authentic case of a child of 

 such a race being born without a prepuce. So also with dehorning 

 of cattle, which is commonly practised in America ; the offspring 

 of such animals are not born polled. The well-known experiments 



