26 THE FIRST PRINCIPLES OF HEREDITY 



produces in its turn, either by division (Aurelia) or by 

 budding (Hydrapolype), once more the sexual free-moving 

 generation. 



The interpolation of asexual reproduction by spores 

 between sexual generations is a well-studied phenomenon 



Fig. 19. — Alternation of Generations in the Common Jelly- 

 fish (Aurelia). (From Plaeckel.) 



{From Geddes and Thomson, " The Evolution of Sex.") 



1 , free-swimming embryo ; 2, embryo settled down ; 3 to 6, the 

 developing asexual stage ; 7 and 8, the formation of a pile of 

 individuals ; 9, their liberation ; 10 and i ij the free-living sexual 

 form. 



among lower plants. In the ordinary Fern we have the 

 big fern-leaf, the asexual plant, producing the familiar 

 spores at the back. These develop when in suitable ground 

 into an inconspicuous green organism, with male and female 

 sex-cells. From the union of both arises once more the 

 tall fern-plant. In the Moss we have a similar process, but 



