70 THE EVOLUTION OF SEX. 



on most of the hind wings the characteristic sulphur of the 

 male prevailed. In other minor ways, the characteristics of the 

 two sexes, which are well marked, were intimately blended. 

 Similar cases are on record. 



§ 5. No7'mal Adult Herinaphroditism. — This is rare among 

 the higher animals, but common among the lower. On the 

 threshold of the vertebrate series, we find it indeed constant 

 among the tunicata ; but above these it is only known to occur 

 normally in two genera of fishes, and in one genus of amphibians. 

 " A testis is constantly found imbedded in the wall of the ovary 

 in Chrysophrys and Serra?ms, and the last-named fish is said to 

 be self-impregnating." In some species of male toad {e.g., Bttfo 

 cinereiis) a somewhat rudimentary ovary is always present in 

 front of the testes. All other cases among vertebrates are 

 either casual (par. 3) or partial (par. 4). Among invertebrates, 

 true hermaphroditism is, however, of frequent normal occur- 

 rence, e.g., in sponges, ccelenterates, worm types, and molluscs. 

 It is necessary to take a brief survey of some of these. 



(i.) Sponges. — As already mentioned, the sex-cells of sponges start 

 simply among the other components of the middle layer {viesoglcea) of the 

 body. It is at least possible' that in any sponge they may develop either 

 into ova or into sperms, or into both, within the same organism, according 

 to nutritive and other conditions. The facts, however, are these. jNIany 

 sponges are only known in a unisexual state, while others are genuinely 

 hermaphrodite. But among the latter it is not uncommon to find {e.g., in 

 Sycandra raphanus) that the production of one set of elements prepon- 

 derates over the other, and thus we have hermaphrodites with a distinctly 

 male or female bias. In other words, they are verging towards unisexuality. 

 It does happen in fact {e.g., in Oscarella lolndaris) that a species normally 

 hermaphrodite may exhibit unisexual forms. It is possible, of course, that 

 in such cases one set of sexual elements may have been wholly discharged, 

 or may even have been overlooked in observation ; but there is no 

 improbability against the supposition, that a preponderance of favourable 

 nutritive conditions might induce a form norm,ally hermaphrodite to become 

 wholly female. This, as we have seen above, is what some believe to take 

 place in the individual histor}- of higher forms. 



(2.) Ccclente-rates. — The members of this class are higher, in ha\ang the 

 production of the sex-cells more restricted, to definite regions, tissues, 

 organs, or even " persons." The highly active Ctenophores, like Beroe, are 

 all hermaphrodite, and that very closely. On one side of the meridional 

 branches of the alimentary canal ova arise, on the other side sperma- 

 tozoa. Among sea-anemones and corals the hermaphrodite condition 

 appears in a number of cases, but is sometimes obscured by the fact that 

 the two kinds of elements are produced at different times, correspondmg to 

 different physiological rhythms in the life of the organism. The genus 

 Coralliiini (the red coral of commerce) is peculiarly instructive. The whole 

 colony may be unisexual, or one branch of the colony, or only certain 



