xvi Introduction, 



the parent cell or germ-vesicle, as suffices to set on foot 

 and maintain the same series of formative actions as 

 those which constituted the individual containing them ' : 

 and this may be taken as the earliest suggestion of the 

 continuity of the germ-plasm. 



Fresh instances of this class of phenomena have 

 steadily accumulated, in which the life-cycle of the 

 species may be represented by two or more generations, 

 differing in form and organization, existing under 

 different conditions, and reproducing themselves in 

 different ways. 



The simplest cyclical rhythm occurs in reproduction 

 by metagenesis, where a sexual and asexual form 

 alternate ; this is the law of development in Medusae 

 and Trematoda. In heterogenesis a sexual and an 

 agamous generation alternate, and in this rhythm the 

 agamous may be juvenile or adult ; in Cecidomyia, for 

 example, the parthenogenetic generation reproduces 

 itself when still immature, while in the Cynipidae, on the 

 other hand, it does so only when perfectly developed. 

 Another variety occurring between one hermaphrodite 

 and one sexual form, is seen in Angiostomum nigro- 

 venosuni, the lung parasite of the frog ; and a still more 

 perfect alternation is found in the thread worm of the 

 snail, Leptoptera appendiculata, where two perfectly 

 formed sexual generations are linked in a cycle. 



Sometimes the sexual or asexual member of the cycle 

 may be complex. The liver-fluke of the sheep gives 

 rise to an active ciliated aquatic embryo, which, after 

 a time, pierces and enters a water-snail to become 

 a passive sporocyst; from its germ-cells rediae^x^ formed 

 within the sporocyst, and after several asexual genera- 

 tions, they give rise to minute cercariae, which leave the 

 snail and creep up the stalks of grass ; here they 



