1893. BIOLOGICAL THEORIES, 215 



tentaculocysts is not a mere community of origin ; the marginal bodies- 

 are vestiges of the rudiments of tentaculocysts, i.e., they are vestiges 

 of tentaculocysts themselves ; but what is their function ? Are they 

 mere vestiges, or are they organs which have undergone a change 

 of function ? 



Again, the coexistence of two genera in the same localities and 

 with the same mode of life shows the way to the answer. They are 

 mere vestiges. The ancestral function is impossible : they cannot 

 serve as rudiments whose function is to develop into organs (tentacu- 

 locysts) useful in a free-swimming phase, because there is no such 

 phase in the life-history. They cannot serve a function of sufficient 

 importance to determine their preservation through Natural Selection, 

 for Lucernaria thrives abundantly without them living in the same way 

 and in the same places. We are therefore driven to the conclusion 

 that these bodies, if not functionless, are at least of so little value to 

 their possessors that the chief competitor in the struggle for existence 

 [Lticevnaria) survives although devoid of them. If this be true, then 

 {see view 2 at commencement of this essay) they must be inconstant 

 (" variable ") in structure, or at least must tend to become so. 



Now this point seemed to be so easy to put to the test of obser- 

 vation that I delayed to publish this essay till I should have an 

 opportunity of applying the test. 



I had not yet had the opportunity of so testing my conclusion 

 when the July number of Natural Science appeared, and in it a 

 record of observations made by my friend, Mr. Hornell, on this very 

 point (vol. iii., p. 33). No communication on the subject had passed 

 between us. His observations were made in ignorance of my con- 

 clusions, and hence the element of bias is happily absent. 



I should have been sanguine indeed to expect so striking a 

 confirmation of my conclusion ! Out of one hundred and eighteen 

 specimens of Halidystus taken in June last in Jersey, seventy-eight 

 (that is, two-thirds of the whole number) exhibited a very marked 

 abnormality in the structure of these organs, i.e., some of the organs 

 were " crowned with tentacles." Of specimens gathered earlier in 

 the year (that is, older specimens which had lived through the winter) 

 a small proportion possessed less than the normal number of these 

 bodies. None were observed to be " crowned with tentacles." 

 Either the tentacle-crowns had been absorbed, or the individuals 

 possessing them had been worsted in the struggle for existence during 

 the past winter. 



Further observation is necessary before we can say which is the 

 true explanation. The fact that many — about 15 per cent, in a lot I 

 received from Messrs. Sinel and Hornell in the spring of the present 

 year — have survived through the winter with less than the normal 

 number of these bodies, shows at least that the possession of the full 

 number of these bodies does not give a very great advantage, if any, 

 to their possessors. 



