THE NATURAL HISTORY OF TUMOURS 563 



manifests its independence by multiplying without limit 

 and living like a parasite upon the parent. In the one case 

 development pursues a normal course. In the other the 

 tissues suddenly exert a power which their racial ancestors 

 used to possess, but which, apparently, in the course of genera- 

 tions has become almost extinct. This they are enabled to do 

 because their development has been arrested. 



The development of the individual is an epitome of the 

 development of the race. This is true not only of the individual, 

 but also of the tissues that compose it. All the tissues of 

 the body, in the course of their development, epitomise the 

 changes that took place in their ancestors during the evolution 

 of the race. If the development of any tissue is interrupted 

 at any point, so that it remains on a plane which was normal 

 for some of its predecessors, that tissue retains and can 

 exercise (supposing conditions are favourable) all the powers 

 those predecessors possessed, no matter how far back they 

 may have been in the evolution of the race. 



The tissues, for example, may be able to exercise the power 

 which was enjoyed by many of their remote ancestors, of 

 throwing off buds capable of independent growth. In those 

 days the buds developed into organs or limbs, or even some- 

 times into complete individuals. Now they can only grow 

 into tumours, shapeless masses, the tissues of which present a 

 general resemblance to those of the parent, capable of unlimited 

 increase, but, unlike the buds of former days, incapable of 

 progressive development, for development has been stopped. 



All that is needed is some exciting cause, such as local 

 irritation, to give the growth a start. 



The influence of interference with development in the 

 production of tumours is well shown by the growths that 

 sometimes originate from embryonic or foetal relics. De- 

 velopment implies not only the progressive advance of tissues 

 that are of use, but no less also the recession and disappear- 

 ance of those that have ceased to be of use. Evolution stops 

 when the power of hereditary transmission fails, but so also 

 does involution. If the progress of development comes to 

 an end, fragments of organs that ought to disappear and 

 become absorbed persist, and continuing to grow and work 

 after a fashion sometimes become the nucleus of colossal 

 tumours. 



