xr.] SPONTANEOUS GENERATION. 367 



without the aid of light. That is the expectation to 

 which analogical reasoning leads me ; but I beg you 

 once more to recollect that I have no right to call my 

 opinion any thing but an act of philosophical faith. 



So much for the history of the progress of lledi's 

 great doctrine of Biogenesis, which appears to me, 

 with the limitations I have expressed, to be victorious 

 along the whole line at the present day. 



As regards the second problem offered to us by 

 Kedi, whether Xenogenesis obtains, side by side with 

 Homogenesis — whether, that is, there exist not only 

 the ordinary living things, giving rise to offspring 

 which run through the same cycle as themselves, but 

 also others, producing offspring which are of a totally 

 different character from themselves — the researches of 

 two centuries have led to a different result. That the 

 grubs found in galls are no product of the plants on 

 which the galls grow, but are the result of the intro- 

 duction of the eggs of insects into the substance of 

 these plants, was made out by Vallisnieri, Reaumur, 

 and others, before the end of the first half of the 

 eighteenth century. The tapeworms, bladderworms, 

 and flukes, continued to be a stronghold of the advo- 

 cates of Xenogenesis for a much longer period. In- 

 deed, it is only within the last thirty years that the 

 splendid patience of Von Siebold, Van Beneden, Leuck- 

 art, Kuchenmeister, and other helminthologists, has 

 succeeded in tracing every such parasite, often through 

 the strangest wanderings and metamorjxhoses, to an 

 egg derived from a parent, actually or potentially like 

 itself ; and the tendency of inquiries elsewhere has all 

 been in the same direction. A plant may throw off 

 bulbs, but these, sooner or later, give rise to seeds or 

 spores, which develop into the original form. A 

 polype may give rise to Medusae, or a pluteus to an 



