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, e¢ 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 
