308 SEXUAL DIMORPHISM 



testes and ducts are developed. The immature males make 

 their way next into the gullet of the female, and there live 

 for some time, probably until they are mature or nearly so, 

 then they migrate to the oviduct. 



The exact steps by which such a condition of things was 

 derived from the doubtless original condition in which the 

 males were similar to the females and independent, can only 

 be inferred from analogy and the possibilities of the con- 

 ditions of life. The young of both sexes abandon the free 

 swimming condition at an early stage, and creep about on the 

 ground. Where the adults are abundant there are probably 

 numerous larvae, and these would have plenty of opportunities 

 to creep over the tentacles of adults, and feed at their expense. 

 The young females would soon abandon such incipient para- 

 sitism, while young males, as soon as their development was 

 advanced enough, would possess the instinct of remaining 

 with the female. The eggs of an adult female would thus be 

 generally fertilised by a male producing sperms for the first 

 time, and the males of the progeny would tend to inherit the 

 immaturity of the father. In some such way it may have 

 happened that the males in successive generations became 

 more precocious and more parasitic, until the present con- 

 dition was attained. 



In the Echinodermata, including star-fishes, sea-urchins, 

 sea-cucumbers and feather-stars, although the sexes are with 

 very few exceptions in separate individuals, external sexual 

 differences, or sexual dimorphism, is generally absent. The 

 reason of this is plainly in accordance with Lamarckian 

 principles, for in this sub -kingdom there is not only no 

 copulation, but no relations between the sexes, and no 

 difference in the conditions under which males and females 

 live. Where one individual of a species can live, others are 

 generally in the neighbourhood, and the male and female 

 reproductive elements, eggs and sperms, are merely discharged 



