36 INHERITANCE, VARIATION AND SELECTION. 



produce seed, but if the seeds are sown they yield only thorny 

 acacias. 



This last case differs from the others in that the thornless char- 

 acter of the acacias appears not to be transmissible. The cases of 

 the fox-terrier which had a graceful action and the grandchildren 

 who were lame like the grandfather are different from the other 

 cases as they relate to functional peculiarities arising from the in- 

 heritance of special structure. 



INHERITANCE BY SEX. 



In the examples of inheritance so far given, characters were 

 transmitted indifferently to any of the offspring. Another kind 

 of inheritance is known as inheritance by sex. In this class come 

 all of those characters which are not connected with the act of 

 reproduction, but which are transmitted to only one sex and are 

 known as secondary sexual characters, and a mass of cases in which 

 the transmission is to both, sexes but more commonly from father 

 to son and from mother to daughter than vice versa. Secondary 

 sexual characters are those which pertain to a particular sex, as 

 a beard on a man and side feathers on the tail of a cock. Although 

 such characters are not transmitted from one sex to the other, they 

 are transmitted through the opposite sex to later generations of the 

 same sex. Thus the beard that characterizes a man will be trans- 

 mitted to the son of that man's daughter, though the daughter 

 herself show not a trace of the beard. The inheritance of second- 

 ary sexual characters is so well known that it is not necessary to 

 dilate upon them. What is necessary to show is that many char- 

 acters, not in any sense sexual characters, tend to be inherited more 

 by the sex in which they originated than by the opposite sex. 



