568 SCIENCE PROGRESS 



the powers those ancestors possessed. The result is the 

 formation of a bud, like the buds that were thrown off from 

 time to time by their ancestors, capable of independent growth 

 and composed of cells, the rate of whose growth and multi- 

 plication depends upon the maturity of the parent-stock at 

 the moment. If the affected cells have all but reached adult 

 age before the interference is felt, the buds that grow from 

 them are all but adult too. The tumour is composed of tissues 

 that resemble those of the normal skin. But if owing to 

 irritation, whether it is mechanical or chemical or due to the 

 action of living organisms, there is a great increase in the 

 proportion of young rapidly growing cells, and the develop- 

 ment of these is checked in their youth, the buds that spring 

 from them resemble them, and then the tumour increases 

 rapidly and spreads wherever it can. 



Tumours, in short, irrespective of their clinical characters, 

 are the product of the innate power of asexual reproduction 

 present in some measure in all tissues, except perhaps the 

 most specialised of all. So long as the development of the 

 tissues continues to be normal, growth and reproduction are 

 normal too. But the development of the tissues is based 

 upon special chemical reactions, which have been evolved 

 in the course of ages, and are still being evolved. If these 

 reactions are interfered with, for example, by some strange 

 element such as arsenic entering into chemical combination 

 with the substance of the cutaneous cells, work is interfered 

 with, so that the skin becomes harsh and dry, development is 

 checked, leaving the surface glazed and rough ; and the 

 young cells that are formed from time to time to replace those 

 that are worn out, instead of attaining their true standard 

 remain upon a lower plane of evolution — a plane that was 

 normal for their ancestors — with the same powers of repro- 

 duction and bud formation that those ancestors possessed, 

 and only require some local stimulus to start them on their 

 career of growth. 



In the illustrations I have given the interference with 

 the nutrition, and the chemical reactions of the tissues has 

 been caused by the slow continuous working of substances 

 introduced into the body from without. Some have entered 

 through the digestive tract, others through the lungs, and 

 others, again, through the skin. Whether substances capable 



