CONSEQUENCES OF ALTERNATE GENERATION. 171 



deny the reality of natural groups, because of these early 

 resemblances, would be to take the semblance for the 

 reality. It would be the same as saying that the frog and 

 the fish are one, because at one stage of embryonic life it is 

 impossible, with the means at our command, to distinguish 

 them. 



361. The account we have above given of the develop- 

 ment, the metamorphoses, and the alternate reproduction of 

 the tower animals, is sufficient to undermine the old theory 

 of Spontaneous Generation, which was proposed to account 

 for the presence of worms in the bodies of animals, for the 

 sudden appearance of myriads of animalcules in stagnant 

 water, and under other circumstances rendering their occur- 

 rence mysterious. We need only to recollect how the 

 Cercaria insinuates itself into the 



skin and the viscera of mollusks, 

 (339, 342,) to understand how 

 admission may be gained to the 

 most inaccessible parts. Such be- 

 ings occur even in the eye of many Fig. 145. Fig. 146. 

 animals, especially of fishes ; they 



are numerous in the eye of the common fresh-water perch 

 of Europe. To the naked eye they seem like little white 

 spots, (Fig. 145;) but when magnified, they have the form 

 of Fig. 146. 



362. As to the larger intestinal worms found in other 

 animals, the mystery of their origin has been entirely solved 

 by recent researches. A single instance will illustrate their 

 history. At certain periods of the year, the Sculpins of the 

 Baltic are infested by a particular species of Tsenia or tape- 

 worm, from which they are free at other seasons. Mr. Esch- 

 richt found that, at certain seasons, the worms lose a great 

 portion of the long chain of rings of which they are com- 

 posed. On a careful examination, he found that each ring 



