CONSEQUENCES OP ALTERNATE EEPEODTJCTION. 351 



s, than the study of their different phases. In fact, a differ- 

 ence so wide as that between the adult Medusa and the 

 adult Campanularia must have existed even in the young; 

 only it does not show itself in a manner appreciable by our 

 senses ; the character by which they subsequently differ so 

 much, being not yet developed. To deny the reality of na- 

 tural groups, because of these early resemblances, would be to 

 take the resemblance for the reality. It would be the same as 

 saying that the frog and the fish are identical, because at one 

 stage of embryonic life it is impossible, with the means at our 

 command, to distinguish them. 



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

 ment, the metamorphoses, and the alternate reproduction of 

 the lower 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 occurrence 

 mysterious. We need only recollect how the Cercaria in- 

 sinuates itself into the skin and the viscera of mollusca ( 520, 

 521), to understand how admission may be gained to the 

 most inaccessible parts. Such beings occur even in the eye 

 of many animals, especially of fishes ; they are numerous in 

 the eye of the common fresh- water perch of Europe. 



544. 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 Tcenia, or tape- 

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

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

 portion of the long chain of rings of which they are composed. 

 On a careful examination he found that each ring contained 

 several hundred eggs, which, on being freed from their enve- 

 lope, float in the water. As these eggs are innumerable, it 

 is not astonishing that the sculpins should occasionally swallow 

 some of them with their prey. The eggs, being thus intro- 

 duced into the stomach of the fish, find conditions favourable 

 to their development ; and thus the species is propagated, and 

 at the same time transmitted from one generation of the fish to 

 another. The eggs which are not swallowed are probably lost. 



