240 CRITIQUES AND ADDRESSES. [x. 



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 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 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 remark- 

 able approximations to true Xenogenesis. 



As I have already mentioned, it has been known since 

 the time of Vallisnieri and of .Reaumur, 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, and the corn 

 are parts of the living body, which have become, to a 

 certain degree, independent and distinct organisms. 

 Under the influence of certain external conditions, 

 elements of the body, which should have developed in 

 due subordination to its general plan, set up for them- 

 selves and apply the nourishment which they receive to 

 their own purposes. 



From such innocent productions as corns and \varts, 

 there are all gradations to the serious tumours which, by 

 their mere size and the mechanical obstruction they cause, 

 destroy the organism out of which they are developed ; 



