258 BIOGENESIS AND ABIOGENESIS vill 



bladderworms, and flukes continued to be a 

 stronghold of the advocates of Xenogenesis for a 

 much longer period. Indeed, it is only within the 

 last thirty years that the splendid patience of Yon 

 Siebold, Van Beneden, Leuckart, Kiichenmeister, 

 and other helminthologists, has succeeded in 

 tracing every such parasite, often through the 

 strangest wanderings and metamorphoses, to an 

 egg derived from a parent, actually or potentially 

 like itself; and the tendency of inquiries else- 

 where 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 Echinoderm, but 

 the Medusa and the Echinoderm give rise to eggs 

 which produce polypes or plutei, and they are 

 therefore only stages in the cycle of life of the 

 species. 



But if we turn to pathology, it offers us some 

 remarkable approximations to true Xenogenesis. 



As I have already mentioned, it has been 

 known since the time of Vallisnieri and of 

 Re'aumur, that galls in plants, and tumours in 

 cattle, are caused by insects, which lay their eggs 

 in those parts of the animal or vegetable frame of 

 which these morbid structures are outgrowths. 

 Again, it is a matter of familiar experience to 

 everybody that mere pressure on the skin will 

 give rise to a corn. Now the gall, the tumour, 



