308 A TEXTBOOK OF BOTANY [Cn. VI, 8 



their sexual differentiation, though the higher animals have 

 gone a little farther, for in them the male and female 

 sex cells are always borne by different individuals, which 

 are distinguished, not only by their very different sexual 

 organs, but also by correlated differences in habits, occupa- 

 tions, dispositions, aspect, stature, and other visible features. 

 Thus, in summary, there runs throughout all sexual dif- 

 ferentiation the one constant thread of the fiisiojT_jrFjhhe 

 two sex cells, which brings together the parental chromo- 



somes in equal contribution to the constitution of the off- 

 spring. It is only the mechanisms subsidiary thereto which 

 vary. These mechanisms originate in a way to imply that 

 the sexes were originally alike ,^and the differences between 

 them arose through ^division of labor, at first between the 

 *ex cells and later between the individuals which produce 

 them, in connection with two subsidiary features of sexual 

 reproduction, viz. effecting the union of the sex cells, and 

 nourishing (and later protecting) the embryonic offspring. 

 Even in the highest plants and animals, sex seems to mean 

 no more than this difference, developed to such a degree as 

 to produce structures, organs, and even individuals, fitted 

 to the respective parts taken by the sex cells. It is indeed 

 possible that other factors are also involved in the result, 

 but if so, they are obscure, while these are obvious. 



8. HEREDITY, VARIATION, AND EVOLUTION 



The matters considered in the preceding sections lead 

 naturally to others expressed in the title of this section. 

 They are largely of theoretical, though very fundamental 

 nature. Although in the past largely speculative in treat- 

 ment they are now the subject of profound experimental 

 researches, the conclusions of which apply equally to plants 

 and animals. 



^Heredity is the resemblance of an individual to its an- 

 cestors,. ^Variation is the difference between an individual 

 and its ancestors? It is easy to see how, granting the chro- 



