204 PARASITISM 



IV. PROTOZOA AND THE CANCER PROBLEM. 



Before describing, in the following chapters, the well-defined and 

 accepted pathogenic protozoa, it may be well to consider first some 

 pseudoprotozoa that have been brought forward from time to time as 

 the cause of cancer. This disease, more than any other human ail- 

 ment, has been a fruitful field for such forms, and the many struc- 

 tures that have been described as protozoa must be regarded only as 

 monuments to innumerable well-meant but immature efforts to dis- 

 cover the cause of this subtle malady. 



Of the many varieties of tumor occurring in man, carcinoma, or 

 "cancer," is the one offering the most striking biological phe- 

 nomena, although there is reason to believe that other tumors, espe- 

 cially sarcomata and epitheliomata, are manifestations of the same or 

 of similar causes. In all cases, whether benign tumor or malignant 

 growth, the one common characteristic is the power of cell prolifera- 

 tion, and the "cancer problem" which today engages the best effort 

 of many pathologists, chemists, biologists, and medical men in general 

 in every civilized country is to ascertain the cause or causes under- 

 lying such proliferation. Many believe that the secret is bound up 

 with the problem of life itself, and will be solved only when the latter 

 is an open secret; but the great majority of investigators fortunately 

 take the more hopeful view that cancer, being an abnormal growth, 

 has some specific and demonstrable cause. 



In every type of animal, including even the protozoa, there is a 

 more or less well-defined power or "potential" of division energy of its 

 cells, a power which gradually diminishes with advancing age and 

 ultimately gives out (see p. 134). The individual cells then cease to 

 multiply, and in the higher animals their activities are directed toward 

 the one physiological object for which they are specialized, and divi- 

 sion is resumed only when some external and unforeseen cause, such 

 as a wound, starts up the inhibited development. Even this power of 

 regeneration is lost to some types of physiologically unbalanced tissue 

 cells. 



In the higher animals the cells of the epithelial group retain the 

 physiologically balanced condition longer than any other type. This 

 is the group to which the germ cells and the endothelial and secreting 

 cells belong, the so-called "noble" cells of the body, some of them, 

 like the skin cells, retaining their division energy throughout life, 

 while others, the germ cells, possess the potential of endless existence. 

 Even among the cells of the epithelial type the potential of division 

 energy varies, and in the highly specialized and physiologically unbal- 

 anced tissue cells it is early exhausted. It is in connection with these 

 cells that we must look for the cause of carcinoma; in their vital 



