422 w - K. BROOKS ON THE LIFE-HISTORY 



Those which were set free either fastened themselves like Hydra, or they remained float- 

 ing in the water, and gradually became adapted to a free life, and thus furnished the in- 

 itial point for the formation of the medusa, which as Koch clearly shows, as Claperede 

 had also shown years before ( 13), is not essentially different from such a hydroid asTu- 

 bularia. In a species with both sessile and swimming persons, the latter, if both were 

 sexually mature, would be much less likely to interbreed closely than the former, and the 

 sessile forms would therefore gradually lose the power of sexual reproduction, while the 

 swimming forms would become the reproductive persons of the species. This speciali- 

 zation of the reproductive function would tend to secure cross-fertilization and it would 

 therefore become established on account of this advantage. 



lie then supposes that some of these hydroids with free medusa? became established 

 in places where the locomotor medusae were exposed to the danger of being swept out 

 to sea, away from proper localities for the attachment and growth of the sessile hydras, 

 before these were born. Natural selection would, under such circumstances, lead to the 

 preservation and perpetuation of those medusas which reproduced their young very early 

 in their own life, and we should thus gradually obtain medusas which became sexually 

 mature before they were detached; and as these medusa' would derive no advantage 

 from a locomotor life they would gradually become converted into sessile medusa-buds. 



According to Leuckart, Gegenbaur, Hamann, Balfour and others, the locomotor or- 

 gans have been acquired for the purpose of distributing the species, and the free persons 

 first became the sexual members of the species, and then became locomotor, while Koch 

 believes that the locomotor life has not been acquired for the purpose of distributing the 

 species; and that the free persons became locomotor medusas before they became special- 

 ized for reproduction, and that the reason for this specialization was not the need for dis- 

 tributing the species, but the advantage of crossing. So far as simple plausibility goes, 

 this hvpothesis is certainly a little more satisfactory than any of the others, but the test 

 of the truth of an hvpothesis is an appeal to fact, rather than its neatness. 



While the various writers who have advocated the hypothesis of polymorphism differ 

 so greatly in their accounts of the process by which the medusa has been evolved, by the 

 specialization of persons detached from a hydroid community, the " proofs " which they 

 advance in support of the belief that it has originated by such specialization are essen- 

 tially the same. As the first step in the argument, they point out the homology between 

 the hydra and the medusa, and justly claim that this proves that both are modifications 

 of some common type, and they then proceed to make either on words or by implication 

 the further assumption that this type must have been either a member of a hydroid com- 

 munity or a medusa. Thus K >eh says, r( in the attempt to study the relation between the 

 hydra and the medusa in the light of the theory of descent, we have to decide between 

 two hypotheses; first, that the medusa is primitive and that the hydroids are only larva 1 

 which have been independently modified, or second, that the hydroids are primitive and 

 that the medusa 1 have been derived from them. The fact that the hydra bears a close 

 resemblance to many other organisms, such as sponges and corals, while the medusa 

 shows no such resemblance to other groups, leads us to reject the first hypothesis and to 

 adopt the second alternative." If we were compelled to accept one or the other of tin se 

 alternatives, there is no doubt that this reasoning would be of great weight, but it is 



