72 ANIMAL INDIVIDUALITY [OH. 



process. These observations show that the sexual act 

 stood originally in no relation to the life of the cell, 

 or of the multicellular organism, or of the race, so 

 that any conclusions with regard to individuality 

 based on the periodical recurrence of sexual fusion 

 cannot be fundamentally true. 



But though the theory cannot be upheld in its 

 entirety, yet some of the facts upon which it is founded 

 are of considerable interest not only generally but 

 also in reference to individuality. 



To start with, the upholders of this theory, such as 

 Professor T. H. Huxley (8 a), base themselves largely 

 upon the facts of metamorphosis, that sudden change, 

 from the grub to the fly, from the tadpole to the frog, 

 that occurs at a definite point in the life of so many 

 animals. What is perhaps the most remarkable 

 example of metamorphosis, that of the Pilidium into 

 the Nemertine, he does not mention, since it was only 

 established some three years later, but as it illustrates 

 his contentions better than any of his own examples, 

 it may be given here. 



Many of the nemertines salt-water worms with 

 long cord-like bodies lay eggs each of which develops 

 into a transparent free-swimming creature, very 

 unlike its parent, and called Pilidium from its re- 

 semblance to a little hat (Fig. 5). The hat is provided 

 with ear-flaps, and between the flaps there is a mouth 

 leading up into a capacious stomach. 



