368 LAY SERMONS, ADDRESSES, AND REVIEWS. [xv. 



Echinoderin, "but the Medusa and the EcMnoderm 

 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 re- 

 markable 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 ani- 

 mal or vegetable frame of which these morbid struc- 

 tures 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 tu- 

 mour, and the corn, are parts of the living body, which 

 have become, to a certain degree, independent and dis- 

 tinct organisms. Under the influence of certain ex- 

 ternal conditions, elements of the body, which should 

 have developed in due subordination to its general 

 plan, set up for themselves and apply the nourish- 

 ment which they receive to their own purposes. 



From such innocent productions as corns and warts, 

 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 de- 

 veloped ; while, finally, in those terrible structures 

 known as cancers, the abnormal growth has acquired 

 powers of reproduction and multiplication, and is only 

 morphologically distinguishable from the parasite 

 worm, the life of which is neither more nor less close- 

 ly bound up with that of the infested organism. 



If there were a kind of diseased structure, the histo- 

 logical elements of which w^re capable of maintaining 

 a separate and independent existence out of the body, 

 it seems to me that the shadowy boundary between 



