424 BIOLOGY: GENERAL AND MEDICAL 



possibly be found in the comparatively undifferentiated 

 condition of the embryonal tissues, for the same tissue 

 died at once when transplanted to the adult fowl, even 

 after several generations of growth in the embryo chick. 

 Unfortunately, these successes have not yet thrown as 

 much light upon the etiology of tumors as they have on 

 the matter of blood relationship and tissue affinities. 

 We have confirmed the fact that the tissues of different 

 animals will not agree, but we have not learned the nature 

 of tumors. 



The question may be asked why the tumor tissue 

 behaves differently from the normal tissue when trans- 

 planted, for we remember that even in the homologous 

 and autoplastic transplantations of normal tissues the 

 grafts are usually subject to decadence, death, and ab- 

 sorption. The answer seems to be found in the abnor- 

 mal impulse of growth that characterized the tumor 

 tissue and stamps it as such. It is tissue that would 

 grow unrestrictedly in its normal environment, and 

 continues to do so when transplanted. 



When we come to consider the conditions of successful 

 grafting in the vegetable world we find that with certain 

 exceptions they form a parallel with what has already 

 been found in the animal world. That is, their success 

 or failure depends chiefly upon the blood relationship 

 of the plants concerned, though this restriction is not 

 so closely defined as among animals. Plants of the 

 same species can, other things being equal, easily be 

 grafted upon one another; plants of the same species, 

 but of different varieties can usually be grafted one upon 

 the other; plants of different species can sometimes be 

 grafted one upon the other; plants of different genera can 

 rarely be drafted upon one another, and after the generic 

 line is past, attempts made to graft individuals of differ- 

 ent families and orders, invariably fail. 



Grafting among plants has been practised from an- 

 tiquity. How the idea originated or why it was origi- 



