KEPKODUCTION AND HEREDITY 287 



Frequently self-fertilization is made impossible by the fact that 

 both kinds of sex-cells mature at different times, so that these 

 animals either function first as males and then as females, or 

 first as females and then as males. The advantage of herma- 

 phroditism is, however, evident, in spite of the avoidance of 

 self-fertilization, especially as regards slow-moving animals, for 

 whenever two individuals meet it is always male and female that 

 meet. The great value placed by Nature on cross-fertilization 

 becomes quite clear when we find that even in many herma- 

 phrodites as, for instance, the duck-mussels there appear at 

 certain times dwarf adjunct males, in the event of mutual impreg- 

 nation of the hermaphrodites being for some reason or other 

 rendered impossible. 



With the numerous other arrangements which aim at a 

 union of the sexes for the purposes of impregnation I can deal 

 here only in very brief outlines. Only in the higher vertebrates 

 do we observe the kind of union which may be described as a 

 marital association, but in the lower forms the union of the sexes 

 occupies only a short period. 



Very remarkable conditions are observable in a species of 

 vermes, Bonellia viridis, found in the Mediterranean. The 

 female lives concealing its sac-like body among the rocks, only 

 the cephalic lappet, which is about 20 to 30 centimetres long, 

 being visible ; the minute male, which is only about 1 millimetre 

 long, and was only quite recently discovered, leads an unnoticed 

 existence in the sexual organs of the female. Similar arrange- 

 ments are found in some of the parasitic crustaceans. 



In Diplozoon paradoxum, a trematode which is parasitic on 

 the gills of carps, both sexes are found grown together like the 

 celebrated Siamese twins. In another species, Schistosomum 

 (Bilharzia) hcematobium, a parasite of man dreaded in Southern 

 countries, the male by rolling up the lateral margins of its body 

 forms a partly closed tube, the canalis gyncecophorus, in which 

 it carries the more slender female through life. These cases are, 

 however, as much exceptions as the symbiosis of many bark- 

 beetles and land crabs, for generally the sexes separate 

 immediately after fertilization if, as in the majority of insects, 

 they do not die shortly after having fulfilled this all-important 

 task. 



