1893. BIOLOGICAL THEORIES. 213 



a consideration of the evolution of tentacles, and of tentaculocysts, 

 and of the marginal bodies of Haliclystus. 



The evolution of the marginal bodies in Lucevnavia offers the 

 difficulties already considered, and, therefore, we may not take these 

 bodies as the starting-point in speculating as to the origin of tentacu- 

 locysts. Before this procedure would be allowable, we must account 

 for the origin of the marginal bodies themselves. As this origin is not 

 obvious, we may consider the other organs, tentacles and tentaculo- 

 cysts, first ; and we will begin with tentacles, as these are the simpler. 

 In doing so, however, it is necessary to bear constantly in mind that 

 the more complex is not necessarily a modification of the simpler — 

 the simpler may just as well be a modification of the more complex, 

 or the two kinds of organs may even have been independently evolved. 



The tentacle in its simplest form is a mere outgrowth of the 

 body-wall, identical in histological structure with the adjacent portions 

 of that body-wall. Almost in this simple form tentacles may be seen 

 in Hydra, though Hydra is probably far from being a primitive form. 

 A mere projection of the wall of the animal, provided it be contractile 

 and suitably placed with reference to the " mouth," may well be 

 regarded as useful in the prehension of food. Its usefulness would, 

 moreover, be greater, within certain limits, the greater the length of 

 the process. Every individual possessing such outgrowths would, 

 therefore, be favoured thereby in the struggle for existence and 

 multiplication ; and this advantage would, moreover, always rest more 

 especially with the individuals in which the processes were, within 

 limits, the longest. Hence the evolution of tentacles from mere 

 irregularities of the body- wall can be explained without any assump- 

 tion beyond that of the operation of Natural Selection. 



How a free-swimming medusa was evolved we are not now con- 

 sidering, and fortunately it is not necessary that we should answer 

 the question at present. Suffice it to say that free-swimming medusae 

 exist, and that the evolution of their tentacles in the above-described 

 way offers no difficulty. 



With tentaculocysts the case is different. If we suppose them to 

 be modified tentacles the difficulties vanish ; otherwise the difficulties 

 are, so far as I can see, insurmountable. The slow transformation 

 of a few tentacles out of many so as to better perform a special function, 

 performed, at best, only badly by a tentacle, leaving the remainder to 

 carry on the original chief function of these organs, appears possible ; 

 for the chief changes required are the formation of a depression 

 around the base of the tentacle and subsequently a shortening of 

 the tentacle itself, and these offer no difficulty. Every stage of 

 such an evolution provided only that the medusas are fairly large, 

 and becoming larger, fulfils all the requirements of the theory of 

 Natural Selection ; that is, the transformation is a gradual one, and 

 every stage in it is an advance upon the previous one ; or in other 

 words, every supposed change in the evolution is a minute one ; is one 



