52 REPRODUCTION AND LIFE-HISTORY OF ANIMALS. 



sexual functions in varying degrees within one organism. 

 It may be demonstrable in early life only, and disappear as 

 maleness or femaleness predominates in the adult. It 

 may occur as a casualty or as a reversion ; or it may be 

 normal in the adult, e.g., in some Sponges and Ccelenterates, 

 in many " worms," e.g., earthworm and leech, in barnacles 

 and acorn-shells, in one species of oyster, in the snail, and 

 in many other Bivalves and Gastropods, in Tunicates and 

 in the hagfish. In most cases, though these animals are 

 bisexual, they produce ova at one period and spermatozoa 

 at another. It is, therefore, very rarely (in parasitic flat 

 worms) that the ova of a hermaphrodite are fertilised by the 

 spermatozoa of the same animal. There can be little doubt 

 that the bisexual or hermaphrodite state, of periodic male- 

 ness and femaleness, is the primitive condition, and that the 

 unisexual condition of permanent maleness or femaleness is 

 a secondary differentiation. The cases which we have cited 

 above, may be interpreted as due to persistence of the 

 primitive condition, or as reversions to it. 



(b) Parthenogenesis, as we know it, is a degenerate form 

 of sexual reproduction, in which ova produced by female 

 organisms develop without being fertilised by male elements. 

 In the hypothetical primitive Metazoa there perhaps was a 

 constant parthenogenesis, in which all the germ-cells were 

 alike capable of developing into organisms without any 

 mutual assistance. As we know it, however, partheno- 

 genesis is a degenerate form of the normal sexual process. 

 It is well illustrated by Rotifers, in which fertilisation is 

 not known to occur, while in some genera males have 

 never been found ; by many small Crustaceans whose 

 males are absent for a season; by aphides, from among 

 which males may be absent for the summer (or in artificial 

 conditions for several years) without affecting the rapid suc- 

 cession of female generations ; by the production of drones 

 in the bee-hive,— for the eggs which give rise to these 

 (male) forms are unfertilised. With the exception of this 

 last case and one other, in neither of which parthenogenesis 

 can be said to have established itself thoroughly, all the 

 parthenogenetic ova that have been carefully examined form 

 only one polar body (see p. 56). There is reason for sup- 

 posing that such ova are less exclusively female than those 



