364 V. THE NATURE OF EVOLUTION 



peculiar environment of the regeneration, in which strong factors are 

 at work to develop the primitive structure into differentiated. Simi- 

 larly, chicken sarcoma cells implanted onto chick embryos are reported 

 to be developed into normal cells under the strong influence of 

 organizer. 



The recovery of the original, normal structure is common in 

 benign tumours, but the normal structure which was recovered from 

 the tumour appears to have the memory of the tumour, tending to 

 return again to the tumour structure. Thus, according to Mackenzie 

 and Rous (52), where tumours once were one finds only epidermis of 

 ordinary appearance, yet if tarring is resumed some of them not in- 

 frequently reappear, even after months, and by reapplying tar at in- 

 tervals they may be made to recur again and again, clear proof that 

 some cells in the apparently normal epidermis have retained their 

 neoplastic potentialities, that is, their memories of the tumour struc- 

 ture. Usually each successive period of tarring brings out more 

 growth than before, and they appear very soon, sometimes almost at 

 once instead of after several months. It is as if new tumour cells 

 had been merely waiting for encouragement. The stimulus of wound 

 healing will suffice to make some of them multiply and form tumours. 



The malignancy of the cancer may depend upon its primitive 

 nature associated with the character of the vigorous multiplication at 

 the sacrifice of the host. It seems strange, however, that germ cells, 

 inspite of their primitiveness, fail to exhibit such a malignancy. This 

 is presumably because there is some mechanism by which the ma- 

 lignancy is inhibited, since the malignancy is, of course, unfavourable 

 for the organisms, but the mechanism may be destroyed by the fertili- 

 zation to the revelation of the malignancy, that is, the character of 

 vigorous multiplication. Leucocytes may be looked upon as another 

 group of primitive cells whose malignancy is usually inhibited ; the 

 destruction of this inhibiting mechanism may lead to leucosis. The 

 structural pattern of leucocytes which thus acquire the malignancy 

 can sometimes be transmitted by protoplasm fragments, and conse- 

 quently the disease is considered to be induced by a virus as in avian 

 leucosis. 



2. The Predisposition to Cancer 



Cancers are liable to develop in certain individuals, that is, certain 

 individuals possess the predisposition to develop cancers. This predis- 

 position is inheritable. As mentioned already, predisposition is a 

 memory of the gene, or a genie structure capable of recovering a 



