34 MANUAL OF ZOOLOGY. 



There are various other cases in which parthenogenesis is 

 said to occurj but the above will suffice to indicate the general 

 character of the phenomena in question. The theories of par- 

 thenogenesis appear to be too complex to be introduced here ; 

 and there is the less to regret in their omission, as naturalists 

 have not yet definitely adopted any one explanation of the 

 phenomena to the exclusion of the rest. 



First Law of Quatrefages. — From the phenomena of asexual 

 reproduction in all its forms, M. de Quatrefages has deduced 

 the following generalisation : — 



" The formation of new individuals may take place, in some 

 .nstances, by gemmation from, or division of, the parent-being ; 

 but this process is an exhaustive one, and cannot be carried 

 out indefinitely. When, therefore, it is necessary to insure the 

 continuance of the species, the sexes must present themselves, 

 and the germ and sperm must be allowed to come in contact 

 with one another." 



It should be added that the act of sexual reproduction, 

 though it insures the perpetuation of the species, is very de- 

 structive to the life of the individual. The formation of the 

 essential elements of reproduction appears to be one of the 

 highest physiological acts of which the organism is capable, 

 and it is attended with a corresponding strain upon the vital 

 energies. In no case is this more strikingly exhibited than in 

 the majority of insects, which pass the greater portion of their 

 existence in a sexually immature condition, and die almost 

 immediately after they have become sexually pei;fect, and 

 have consummated the act whereby the perpetuation of the 

 species is secured. 



II. Development, Transformation, and Metamorphosis. 

 Development is the general term applied to all those changes 

 which a germ undergoes before it assumes the characters of 

 the perfect individual; and the chief differences which are ob- 

 served in the process as it occurs in different animals consists 

 simply in the extent to which these changes are external and 

 visible, or are more or less completely concealed from view. 

 For these differences the terms " transformation " and "meta- 

 morphosis " are employed ; but they must be regarded as essen- 

 tially nothing more than variations of development. 



beyond reasonable doubt that the males are produced by a process of par- 

 thenogenesis. _ Landois, however, asserts that the eggs of insects are of no 

 sex, that sex is only developed in the larva after its emergence from the 

 egg, and that in e^ch individual larva the sex is determined wholly by the 

 nature .of the fpod upon which it is brought up; abundant nourishment 

 producing females, and scanty diet giving rise to males. 



