TUMORS. 301 



but at present it seems to us that no adequate ground exists for believing 

 that they are. 1 



It is indeed not improbable that a local predisposition to tumor 

 formation akin in nature to that which now and then is attributable to 

 trauma may be occasionally induced by micro-organisms. But this it 

 remains for future researches to demonstrate. 



The Complexity of the Problem of the Etiology of Tumors. It seems to 

 the writer that to seek for a single external excitant or group of ex- 

 citants for the aberrant tissue growths which we call tumors is to ignore 

 the many still obscure influences which are at work in all tissue growth, 

 especially those physiological agencies which foster cell proliferation and 

 tend, under the influence of heredity, to specialization in form or func- 

 tion. On the other hand, not to be ignored are those influences, whether 

 of nutrition or pressure or exposure, which mould the cell growth under 

 normal conditions into fixed and useful forms. It is rather a matter for 

 surprise that ever-changing, self-regenerating living tissue does not 

 ofteuer go astray in its activities than that it should only now and then 

 do so. This latter somewhat inverted point of view may be useful in 

 calling away the attention, in discussing the etiology of tumors, from a 

 too close regard to extraneous factors, and directing it to the many still 

 unexplored fields in cell physiology with which we must perhaps become 

 familiar before we can with fair hope of success attack the serious prob- 

 lems of both the excitation and the prevention of tumors. 



Xo adequate conception can be gained of the complexity of the prob- 

 lem involved in the origin of tumors or of the directions in which the 

 highest promise of experimental research lies without a recognition of the 

 new cell lore derived from a study of the lower and simpler forms of life. 2 



The more commonly accepted and time-honored view of the imme- 

 diate excitation of tumors is that they are due to some stimulus "forma- 

 tive stimulus " by which the proliferative capacity of the cells is in- 

 creased. "SVhat the nature of this postulated stimulus may be and how 

 it is brought to bear upon the transformations of energy which through 

 the subtle mechanism of the cell is manifested in proliferation, is in 

 nowise clear, nor does it seem likely to become so until we gain a deeper 

 insight into the life processes of the cell in general. It is possible that 

 recent experiments (referred to on page 106) on the artificial excitation 

 by chemical substances of cell proliferation and the development of the 

 embryo in certain of the lower animals may lead to a wider conception 

 of the nature of the conditions under which tumors are formed. 



There are obvious alterations in the character of the cell growth in 

 tumors in which they deviate often widely from the norm. It has been 



1 For bibliography relating to the alleged parasitic excitants of tumors, consult 

 Gilchrist, Johns Hopkins Hospital Reports, vol. i., p. 332, 1896; also- Park, American 

 Journal of the Medical Sciences, vol. cxv., p. 516, 1898. For an interesting resume of 

 the factors involved in the origin of tumors see Adami, vol. vii., pp. 309 and 343, 1901, 

 or Brit. Med. Jour., March 16th, 1901. 



' 2 For a clear and comprehensive summary of the new lines of research in cytology, 

 see Wilson, "Some Aspects of Biological Research," The International Monthly, July, 

 1900. 



