THE NATURAL HISTORY OF TUMOURS 565 



which their working depends have been gradually evolved 

 in the course of ages from simpler ones of the same kind. 

 According to Ehrlich and others, the process of chemical 

 evolution is still going on. New ferments are being formed, 

 even at the present time, to deal with new conditions, and 

 structure and arrangements are modified accordingly. So it is 

 with other tissues and other parts of the body — those, for 

 example, that are concerned with the transmission of impulses 

 from one organ to another. Special work everywhere involves 

 the evolution of special chemical reactions ; and as generation 

 succeeds generation, and the reactions become better adapted 

 to the end in view, the structure and arrangement of the part 

 become modified to suit. 



The development of a tissue or of an organ, in other words, 

 is the outcome of the persistent cultivation of a special series 

 of chemical reactions, brought into existence by the special 

 work assigned to that particular part of the body. At the 

 beginning these reactions were of the simplest character. As 

 time went on and the race was built up, they were built up, 

 gradually, step by step ; and as the individual is an epitome 

 of the race in its chemical reactions as well as in its structural 

 details, so all the chemical stages through which the race has 

 passed are reproduced in the development of the individual, 

 only compressed beyond recognition. The development of 

 the individual is in part the product of the chemical reactions 

 that have taken place in its ancestors from time immemorial, 

 handed down by inheritance from generation to generation, 

 in part the result of the chemical changes that are taking 

 place in its own tissues at the present time. The essential 

 point is that all development, like growth, is ultimately the 

 outcome of chemical changes in the tissues, and that anything 

 that interferes either with the inheritance of the products of 

 past chemical changes, or with the effects of present ones, 

 interferes with the development of those tissues, so that they 

 remain on the same plane as their racial ancestors, and enjoy 

 the same powers of reproduction. 



So little is known of the intimate nature of the chemical 

 changes that take place in the tissues that it is not easy to 

 cite instances in which the failure of any particular reaction 

 has led directly to the cessation of development and the birth 

 of a tumour. There are, however, many isolated facts that 

 37 



