334 THE CANADIAN NATURALIST [Sept. 



the introduction of the eggs of insects into the substance of these 

 plants, was made out by Yallisnierij 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 advocates of Xenogenesis for a much longer period. 

 Indeed, it is only within the last thirty years that the splendid 

 patience of Von Siebold, Van Beneden, Leuckart, Kuchenmeister, 

 and other helminthologists, has succeeded in tracing every such 

 parasite, often throughthe 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 develope into the original form. 



A polype may give rise to Medusae, or a pluteus to an Echiuo- 

 derm, 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 approx- 

 imations to true Xenogenesis. 



As I have already mentioned, it has been known since the time 

 of Vallisnieri and of Beaumur 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 outoTOwths. 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 

 or"-anisms. Under the influence of certain external conditions, 

 elements of the body, which should have developed in due subor- 

 dination to its general plan, set up for themselves, and apply the 

 nourishment which they receive to their own purposes. 



From such innocent productions as corns and warts there are all 

 o-radations to the serious tumours which, by their mere size and 

 the mechanical obstruction they cause, destroy the organism out 

 of which they are developed ; while, finally, in those terrible struc- 

 tures known as cancers, the abnormal growth has acquired powers 

 of reproduction and multiplication, and is only morphologically 

 distinguishable from the parasitic worm, the life of which is neither 

 more nor less closely bound up with that of the infested organism. 



If there were a kind of diseased structure, the histological 

 elements of which were capable of maintaining a separate and 



