228 THE EVOLUTION OF SEX. 



figured above, where two distinct animals {Diplozoon) join in almost 

 lifelong union. 



In many cases again, especially in bony fishes, there is a sexual 

 attraction between male and female, but without any copulation. The 

 female, accompanied by her mate, deposits ova, which he thereupon 

 fertilizes with spermatozoa. A slightly more advanced stage is seen 

 in the frog. Fertilization is still outside the body of the mother, but 

 the male, embracing the female, liberates spermatozoa upon the eggs, 

 which are at the same time laid. 



In the majority of cases, however, special organs for emitting and 

 for receiving spermatozoa are developed, and copulation occurs. The 

 male organ is often an adaptation of some structure already existing, 

 as in many crustaceans, where modified appendages form external 

 canals for the seminal fluid. In skates and other gristly fishes, the 



remarkably complex copulatory organs, 

 so-called "claspers," are in close con- 

 nection with the hind limb. , The penis 

 of higher vertebrates is virtually a new 

 organ. The copulation may be quite 

 external, as in crustaceans, where the 

 male seizing the female deposits sper- 

 matozoa upon the already laid eggs. 

 Fig. ^.-Dipiozoon paradoxnm,* double Oftener, however, it is internal, and the 



organism formed from the i ntrom i ttent organ j s inserted into the 



union or two distinct nerma- ° 



phrodite individual trema- genital aperture of the female. True 



todes (DiporfHi) at an early i • ... , 



stage in their life. copulation may occur without the pres- 



ence of special organs, — notably in 

 the case of many birds, where the cloaca of the male is apposed to that 

 of the female. The spermatozoa, forcibly expelled by the excited male 

 organs, pass up the female ducts, probably, in part, as the result oi 

 peristalsis, but chiefly at least by their own locomotor energy, and one 

 of them may eventually fertilize an ovum. In addition to the intro- 

 mittent organ, and the lower portion of the female duct which receives 

 it during copulation, there may be auxiliary structures, such as true 

 claspers for retaining hold of the females. The limy " cupid's dart" 

 or " spiculum amoris " of the snail, is usually interpreted as a pre- 

 liminary excitant. 



Three further notes in regard to higher animals are requisite, (i) 

 There is much reason to believe that the follicles tend to burst toward 

 the end of menstruation ; that this may be accelerated by copulation ; 

 successful fertilization may occur at any period, but most frequently 

 soon after menstruation, and most rarely during the relatively infertile 

 period most distant from that process. (2) After conception, when 



