38 On the Development of MoUusks in Holothurise. 



being anything but convinced, that the Professor's conclusions 

 are either necessary or even well-founded. 



Prof. Miiller considers that his discoveries have established 

 the occurrence in Synapta digitata of what he calls "hetero- 

 gony/' or " heterogonous generation/' that is, the production 

 by a given species of offspring similar to itself, and of offspring 

 dissimilar to itself, by true sexual generation ; such offspring in 

 each case being able to produce young like itself by sexual ge- 

 neration. Prof. Muller further points out, that this process is 

 very distinct • from the ^^alternation of generations/' and he 

 suggests that it may explain the mode of the introduction of new 

 species upon the surface of our planet. 



There is of course no ^a-priori reason why this " hetero- 

 gony'' should not occur; but it is only reasonable to require 

 that so novel and startling a theory should at least be based 

 upon very strong evidence, and that the insufficiency of any at- 

 tempt at a simpler method of explanation should be clearly de- 

 monstrated. 



Now, we cannot think that Prof. Miiller has done this. Take, 

 for instance, a possibility which he himself suggests (only, how- 

 ever, summarily to dismiss again), that the molluskigerous sac 

 may be the " equivalent of a mollusk — a vermiform metamor- 

 phosis of a mollusk as it were." How much might be said in 

 favour of this supposition ! The mode of development of very 

 few mollusks is yet known, and their parasitism has been still less 

 inquired into. Before the discoveries of Nordmann, who could 

 have anticipated the extraordinary forms in which Crustacea are 

 found parasitic ? Lernceocera and Pennella are as little like crabs 

 as the molluskigerous sac is like a mollusk. 



Again, in considering the probability of a mollusk taking on 

 a worm-like parasitic form, we must not forget that Hectocotylus 

 is the male of the Argonaut, and yet that its form and habits 

 led Cuvier to place it among the worms. 



As to the " grand difficulty for every theory," — the organic 

 connexion of the sac with the Holothuria, — consider some cases 

 of insect parasitism detailed by Dufour (Annals, Nov. 1851), in 

 which there is an "organo-plastic" union between the stigmata 

 of the parasite and those of the insect in which it dwells. 



We do not, of course, offer such analogies as these, as in 

 themselves an explanation of the facts observed by Prof. Miiller, 

 but merely to justify our belief that such an explanation may 

 yet be found without recourse to the doctrine of " heterogony." 



On one subject Prof. Miiller appears to us to be decidedly in 

 error ; we mean in supposing that '^ heterogony " occurs among 

 the Polypes and Medusae. He says, " These genera of Polypes 

 (i. e. Tubularia, Eudendiium, Campanularia, &c.) produce two 



