404 ZOOLOGY. 



The wording of the right-hand column of this comparison 

 is a little too positive : the occasional occurrence of partheno- 

 genesis (chap, xvii., 5), and of " self-fertilization " (i.e. the 

 union of ovum and spermatozoon from the same parent) 

 is sufficient proof that the ordinary "laws" of sexual 

 reproduction in Metazoa express rather what is generally 

 advantageous to a species than what is inevitable; and 

 under peculiar circumstances the advantageousness may 

 disappear. Parallel cases in the higher Protozoa are fur- 

 nished by forms which are able to dispense with conjugation 

 completely, or to a much greater extent than Paramecium. 



