cxiv THE BEGINNINGS OF LIFE. 



pre-existing tissues as are other non-specific morbid growths. 

 A mere local change in the mode and intensity of pre-exist- 

 ing tissue-changes suffices to engender them. In the case of 

 tubercle, this has been conclusively proved by such experi- 

 ments as those of Dr. Burdon Sanderson and Dr. Wilson 

 Fox. The latter says 1 : 'M. Villemin's position, that tu- 

 bercle is a specific disease, producible by tubercle alone, 

 cannot, I think, be held to be true ; nor can the method of 

 inoculation be used as a test of the tubercular character of 

 any pathological product ; for the four guinea-pigs in whom 

 the vaccine lymph was inoculated, and those inoculated 

 with putrid muscle, and even one beneath whose skin I sim- 

 ply inserted a piece of cotton-thread, and also one of the 

 four in which, following Dr. Sanderson's example, I inserted 

 a seton, presented as intense and typical specimens of the 

 disease as those on whom inoculation had been practised 

 with the most typical grey granulations from the lungs or the 

 meningeal vessels.' What has now been (even experimen- 

 tally) established with regard to tubercle, seems also to hold 

 good for such ' malignant ' growths as recurrent-fibroid, epi- 

 thelial, and cancerous tumours or infiltrations. Statistics to 

 which Virchow has drawn prominent attention seem most 

 clearly to indicate the potency of ' exciting causes ' in giving 

 birth to these growths. Are they not found primarily, with 

 by far the greatest frequency, in situations which are exposed 

 to the action of irritative agencies, either external or inter- 

 nal, normal or abnormal? An amount of irritation which 

 in some persons may lead to chronic inflammation or a 

 hyperplasic overgrowth, will in others suffice to produce one 

 of these so-called ' malignant ' growths, even without the aid 

 of any ascertained predisposition. The history of many 

 cases of ' labial cancer,' and of that form to which chimney- 

 sweeps are liable, explains almost as clearly the origin of 

 1 On the Artificial Production of Tubercle,' 1868, p. 23. 



