694 The Outline of Science 



independent halves. A pair of identical twins represent a single 

 human being which chance has decreed shall become two. 



Our previous remarks about the restriction of asexual repro- 

 duction in higher forms thus require a little qualification; for 

 even in the highest groups it may still occur in early stages. The 

 different members of a litter of mammals generally arise from 

 separate fertilised eggs. But in the Texas Armadillo, the quad- 

 ruplets which the female always produces are formed by budding 

 from a single embryo. Here, therefore, identical twins are nor- 

 mal, while in human beings they are accidental, but in both cases 

 they are the result of the power of unlimited reorganisation and 

 growth to be found in the early stages of all animals. 



A consideration of these facts leads us on to other fields. 

 For one thing, there is cancer. In cancer it appears that certain 

 cells of the body escape from the dominating or controlling 

 activity of the rest, and start unregulated growth on their own 

 account. What is more, they seem in some particulars to revert 

 to a more primitive condition, in which their powers of growth 

 and multiplication are increased, and their capacity for perform- 

 ing the ordinary work demanded from the various cells which co- 

 operate in a healthy body is correspondingly impaired. 



What a delicate balance exists in the body of one of the 

 higher animals is shown by the observations of Miss Slye on 

 spontaneous cancer in mice. Some of the female mice which 

 developed the disease were kept separate, while others were mated 

 and allowed to breed. In the former lot the cancer grew at a 

 great rate, and death occurred in about a month. But in the 

 second lot, so long as litter followed litter without pause, the 

 tumour's growth was negligible, to become active, however, as 

 soon as reproduction ceased. In other words, the tumour and the 

 developing embryos were competing for food-substances for their 

 growth, and the embryos were so successful in their demands that 

 they left next to nothing over for the cancer. 



In any event, even though we are far from any proper under- 



