248 EVOLUTION AND DISEASE. 



the significance of this specimen, it should be studied in 

 conjunction with the remarks on the marsupial pouch 

 contained on p. 171, et seq. 



SUMMARY. To epitomise the facts briefly considered 

 in this chapter relative to tumours, from an evolutionist's 

 point of view, it may be stated that (excluding those 

 arising from accidental interference with the function of 

 secreting organs), some tumours arise as a result of 

 change of function in organs, rendering some part of 

 them useless ; others arise from the introduction into 

 an organism of bacteria which have by imperceptible 

 stages become slowly modified so as to be able to thrive 

 on or in animal bodies ; some, and these the most inno- 

 cent forms, arise as "sports" ; whilst the gland tumours and 

 cancers are due to the epithelial modifications which give 

 rise to secreting glands. It may be further stated that 

 animals other than man are liable to tumours, agreeing 

 in all respects with those which have been so long and 

 closely studied in him. Of all tumours occurring in the 

 lower animals, so far as the facts at our disposal show, 

 the commonest forms are the infective granulomata, 

 including sarcomata. 



With respect to the cancer group there are some 

 very extraordinary facts which cannot be passed over 

 in silence. For instance, hair, teeth, feathers, glands 

 of all kinds and cancers arise on fundamentally the 

 same plan, a downgrowth of epithelium into the 

 subjacent tissues ; it is certainly a suggestive fact that 

 abnormal irritation will produce a crop of hairs in an 

 unusual situation, as was shown in an early chapter, 

 and used to explain the curious hairs at the pyloric end 



