THE BEGINNINGS OF LIFE. 



to that which the transplanted fragment of epidermis exerts 

 upon the ulcerated surface. This becomes covered, not so 

 much by an actual increase of the imported fragment as by 

 the formative changes which its presence incites. A crystal 

 thrown into a mixed solution of saline substances will also 

 determine, by its mere presence, the crystallisation of similar 

 materials from the solution; nay, it may determine, in addi- 

 tion, the crystallisation of other products whose modes of 

 aggregation are more or less similar (isomorphous salts). 

 The contact of any number of germinal particles with the 

 tissues of an organ will not produce the formation of a new 

 growth, unless the molecular actions (or modes of growth) 

 existing in the part are such as to make the transition an 

 easy one. The mere presence of ' germs,' therefore, is not 

 all that is necessary. Cancerous masses may grow into the 

 vena cava, and yet no cancer may spring up in the lungs : the 

 stomach may be absolutely infiltrated with cancer, and yet, 

 as I have recently seen, no similar growths may exist in the 

 liver \ Detail, however, is needless on such a subject. The 

 distribution of morbid growths throughout the body, as is 

 well known, takes place, if at all, in a manner so irregular in 

 different individuals, as to make the result wholly beyond 

 the possibility of predication. Having to do with a case of 

 syphilis, who would venture to fix upon the internal organs 

 which would become affected? Who can account for all 

 the irregularities observable in the generalisation of tubercle 

 either in man or in the rodent animals 1 When cancer exists, 

 who will affirm which organ shall be secondarily affected, 

 and which not.? 



^ We find, therefore, that in the absence of any apparent ' predisposition,' 

 exciting causes, when potent, are sufficient to determine the occurrence 

 of secondary growths, just as other exciting causes seem capable of de- 

 termining the primary growth. As the exciting causes are weakened, the 

 new formations occur only under the influence of a predisposition ^he- 

 reditary or acquired), or of a natural aptitude on the part of the tissue to 

 lapse into the morbid mode of growth. 



