108 THE ALTERNATING GENERATIONS 



until they have attained their full development ; in others 

 they forsake them in an earlier condition, are free and 

 free swimming, and undergo a complete metamorphosis ; 

 in some of these latter, the earlier generations are trans- 

 formed into motionless and as it were Hfeless cysts, whilst 

 in others, they remain free and active, (the " nurses' and 

 ''^parent nurses' of Cercaria ephemera and C. echinata,) 

 but retain during their whole life a form, which, at most, 

 resembles the larvae of the more perfect generation. In 

 this way an advance in a certain direction, may indis- 

 putably be observed ; at first, all the generations consti- 

 tute a unity, not merely as regards the interior, but also 

 with respect to the exterior ; they form a stationary 

 colony ; after which, the generations are detached more 

 and more from each other, and become at the same time 

 more free ; and finally, all the individuals constituting the 

 generation are separate from each other, and acquire the 

 power of free locomotion ; in this latter stage, or that of 

 freedom and perfection, we found the development of 

 animals which are certainly no longer attached to inanimate 

 objects at the bottom of the sea, but live buried in other 

 animal organisms, and belong not to the sea, but to fresh 

 water. In a stiU higher and more free stage than this, we 

 observe the development of animals, which do not belong 

 to the water, but to the air, as in that which occurs in the 

 ApJddes. The propagation of these creatures through a 

 series of generations has been already long known. In the 

 spring, for instance, a generation is produced from the ova, 

 which grows and is metamorphosed, and without previous 

 fertilization, gives birth to a new generation, and this 

 again to a third, and so on, for ten or twelve weeks ; so 

 that in certain species, even as many as nine, such pre- 

 liminary generations will have been observed ; but, at last, 

 there always occurs a generation consisting of males and 

 females, the former of which, after their metamorphosis, 

 are usually winged ; fertilization and the depositing of 

 eggs takes place, and the long series of generations re- 

 commences in the next year, and in the same order. All 



