110 Prof. Miiller on the Production 



occur simultaneously would differ entirely from the alternation 

 of generations, and is self-contradictory. This, however, is by 

 no means necessarily to be assumed in our comparison, since an 

 aberrant generation may begin by gemmation. The complete- 

 ness of the parallel depends essentially upon whether we consider 

 two classes, like Polypes and Medusae, whose members alter- 

 nately arise in the course of the ' alternation/ as definite and 

 separate classes, or whether they do not rather form but one 

 class. If they be not two distinct classes of the animal kingdom, 

 all ground for a comparison of our case with an alternation of 

 generations, viz. an alternation between Holothuria and mol- 

 lusks, ceases ; for these are unquestionably separated by a much 

 wider gap than Medusae and Polypes. 



" It is now worth while, concisely, to sum up the various pos- 

 sibilities. The dilemma is, either the molluskigerous sac is itself 

 an animal, or it is an organ of the Holothuria. In the one altex'- 

 native, as in the other, we have to deal with phenomena of the 

 strangest kind. If the sac be an animal, a worm, not produced 

 by the Holothuria, but arisen from a mollusk, we have perchance 

 a quite unexpected case of the alternation of generations. To 

 believe this would perhaps be the easiest mode of reconciling 

 ourselves to facts so anomalous. We have become accustomed 

 in this region to much that is wonderful and yet has accommo- 

 dated itself to the law, so that we must not readily be startled. 



" Or there is no alternation, but a metamorphosis takes place. 

 The mollusk metamorphoses itself into a parasitic worm, which 

 again produces mollusks — a wholly unexpected but by no means 

 an irrational supposition. 



" If the sac be a worm produced by the Holothuria, then it is 

 much more incomprehensible and wonderful, and surpasses any 

 conceivable mode of ' alternation/ 



" If the sac be no animal, no worm, but an extraordinary organ 

 of the Holothuria, it is still perfectly inexplicable, and becomes 

 an ultimate fact by which other facts may be explained." 



We can but remark here, that the want of a clear distinction 

 between gemmation and generation, between an animal and an 

 organ, appears to produce a singular confusion of ideas. 



" That two sexual generations of different kinds should occur 

 simultaneously " certainly differs entirely from any known u al- 

 ternation ; " but why it should be " self-contradictory," we are at 

 a loss to discover. However improbable such an occurrence may 

 be, it is difficult to see why it should be impossible. Besides, 

 Prof. Miiller does assume it to occur, if he considers that the 

 molluskigerous sac is produced within the Synapta. Supposing 

 that sac to be produced by gemmation within the Synapta, is it 

 any the less an organ of the Synapta ? If it be, then the water- 



