142 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



and to make its potential superiority actual. In adult rana pipiens, after 

 transplantation of autogenous white skin to a pigmented dorsal area, no 

 change in the condition of the transplant takes place, but following homoio- 

 transplantation, the pigmented epidermis invades the white epidermis after 

 about two weeks (H. H. Vogel). 



A related process may take place at the border between the squamous 

 epithelium of the cervix and the cylindrical epithelium of the uterine horns. 

 This is especially noticeable in the mouse under the influence of stimulation 

 of the vagina-cervix-uterus by estrogen. Under these conditions the squa- 

 mous epithelium dominates over the cylindrical epithelium and begins to in- 

 vade and replace it ; it may also push into uterine gland ducts and here un- 

 dermine or exert pressure on the cylindrical epithelium. This invasion may 

 extend to various distances. There are indications that such changes may take 

 place to a slight extent even without the hormone stimulation, but it is the 

 latter which greatly intensifies the potential superiority of the squamous epi- 

 thelium of the cervix over the cylindrical epithelium of the uterus. 



In certain respects, also, cancerous growth may be considered as a related 

 phenomenon. In this condition one tissue, as a result of long-continued stim- 

 ulation, gains the ascendancy over adjoining tissues of the same or of a 

 different kind and then begins to invade them. But in cancer such a change is 

 not temporary, as in the examples previously mentioned; it is a permanent 

 change, leading ultimately to the destruction of the whole organism. The 

 mode of stimulation of one tissue which brings about this result in cancer 

 may be of various kinds, but this is the less important factor ; it is the reac- 

 tion of the stimulated tissue which is characteristic. Thus we may conceive 

 of an organism as an equilibrized system, composed of many mosaic parts, 

 which function in harmony with one another. Various kinds of changes may 

 disturb their equilibrium and then a potential tissue superiority may become 

 actual. Tissue differentials, without the co-operation of individuality dif- 

 ferentials, may condition such disharmonious reactions, but antagonistic, dis- 

 equilibrizing reactions may be induced also by individuality differentials, 

 and in some cases they are brought about by an interaction between tissue 

 and individuality differentials. 



