286 LECTUEES ON BIOLOGY 



and tunicates ; and even among the highly developed vertebrates 

 cases of hermaphroditism exist in the Myxine glutinosa and 

 Serranus scriba. 



We are therefore compelled to assume that the hermaphroditic 

 development of animals represents the original stage from which 

 in the course of their phylogenesis a separation of the sexes 

 was gradually evolved. This assumption is supported in parti- 

 cular by the fact that even in vertebrates and insects, in which 

 normally the sexes are always separate, hermaphroditism occurs 

 occasionally abnormally. As these animals frequently possess 

 a prominent sexual dimorphism, it sometimes happens that 

 striking malformations appear. Thus we know of ants, bees, 

 butterflies, beetles, &c., in which the right body-half is purely 

 male and the left purely female, or vice versa. In the Berlin 

 Natural History Museum is preserved the skin of a bull-finch 

 in which the dividing line of the body is strictly vertical, so 

 that one breast half exhibits a bright red male plumage while 

 the other half shows the insignificant grey of the female. As 

 unfortunately only the skin was handed in, it was impossible 

 to examine the inner organization in this bird. This is all the 

 more regrettable as hermaphroditism frequently exists only 

 externally, the sexual glands of animals having in spite of it 

 retained their purely female or purely male character. 



In some cases it seems clear that the bi-sexual forms have 

 acquired their hermaphroditism only secondarily, and that separa- 

 tion of sexes had already taken place in their ancestors. The 

 cause of this lies in a change of the life-habits, and rests either, 

 as in the fly Termitoxenia and numerous other animals, on 

 adaptation to a parasitic mode of life, or, as in the Cirripedia, 

 in the loss of free motility. 



The degree of hermaphroditism frequently varies. As a rule 

 ovaries and testes are found in the same individual, but in 

 different places, possessing separate outlets for the sex-products. 

 Only in very few cases as, for instance, in some vermes, 

 echinoderms and mussels, as well as in numerous snails male 

 and female germ-cells originate generally in the same gland, 

 which is in that case described as the hermaphroditic gland. 

 But even in this extreme case Nature seeks to prevent self- 

 fertilization, permitting it only in cases of extreme necessity. 



