2IO NATURAL SCIENCE. Sept.. 



of " heredity," and is older than " heredity," and has led to the 

 evolution of "heredity" from "variability," or to adopt the 

 safer terms, the evolution of constancy from inconstancy. 

 Inconstancy is the older : constancy which is never complete is 

 only inconstancy kept within narrow limits by the continued 

 action of Natural Selection. The assumption that "heredity" 

 occurred ab initio, and that "variation" has been produced by 

 various external agents — Lamarckian and others — is a mistake 

 (vol. i., p. 580). 



4. The ontogeny of an individual, though in some cases it 

 presents a marked resemblance to the real or supposed phylogeny 

 of the species, with reference to certain characters, usually shows 

 only a very slight such resemblance, and may show none at all. 

 We are, therefore, not justified in relying upon it as a " record" 

 of the phylogeny, even though we admit that the supposed record 

 is highly imperfect. Von Baer's law is readil}^ verified in some 

 cases at least ; and while it appears to contain all that is true in 

 the " Recapitulation " Theory, it is contradictory to the rest of 

 that theory (vol. ii., pp. 197 and 365, and especially 368). 



5. The resemblance between an adult animal on one hand and 

 the larva of another species on the other hand, while indicating 

 a close relationship between the two species, does not show that 

 the second is descended from the first. A stalked crinoid may 

 resemble the immature form of a free-swimming one, and the 

 latter may be actually descended from a form very like the 

 former, but the resemblance in question does not alojie justify this 

 conclusion, inasmuch as the same resemblance might have arisen 

 from a relationship the direct reverse of that supposed {Locc. 

 ciU.) The following essay treats of a case in which this reverse 

 relationship appears to be very near the true one. 



6. Tentaculocysts cannot subserve an auditory function ; and 

 such a function would be useless to a medusa if they could 

 (vol. ii., p. 353, last paragraph) ; but they may and probably do 

 serve a function of the utmost importance in the preservation of 

 jelly fishes from destruction by oceanic waves (vol. ii., p. 421). 

 While writing the essay last referred to, the following question 



occurred to me with almost maddening persistency : — Haliclystus has 

 tentaculocysts or something of the kind and does not swim : can it then he that 

 the function suggested is the true one, seeing that such organs are present also in 

 an animal, to which such a function would be useless ? 



It was not till I had answered that question in my own mind that 

 I ventured to publish the above-mentioned view on the " true function 

 of tentaculocysts." The reason for withholding my answer till now 

 will appear later. 



The structure of the marginal bodies of Haliclystus [Lucernavia) 

 ■octoradiatus shows that they are unfitted for the performance of the 



