22 THE FIRST PRINCIPLES OF HEREDITY 



stamen only or pistil only. When uni-sexual flowers of 

 both kinds, male and female, are to be found on the same 

 plant, the plant is said to be " mon-oecious '\(as oak, alder, 



FIG. 1 6. CATKINS OF WILLOW (SALIX). 



(From Oliver, " Elementary Botany.") 



A, male ; B, female. 



hazel) ; when male and female flowers are on different 

 plants, such plants are " di-cecious " (as in the willows). 



Copulation. 



In order that the germ-cells, male and female, may reach 

 each other, the most varied contrivances exist in Nature. 

 Darwin has described most wonderful adaptations of 

 flowers for the purpose of attaining fertilization, either by 

 the agency of the wind or of insects. 



In the animal kingdom the means of bringing together 

 male and female germ-cells, which here are called sperma- 

 tozoon and ovum respectively, are in the lower classes of 

 extreme simplicity. The liberated sex-cells meet each 

 other in a random manner, brought together by water- 

 currents, etc., either within the body-cavity (sponges) or 

 outside it (as in the sea-squirts). There is, it is true, a 

 subtle chemical attraction between the generative elements, 



