242 EVOLUTION AND DISEASE. 
a large size. Such a tumour of a tree is termed a 
xyloma. The bud-like character of such woody tumours 
is shown in an interesting series presented to the 
museum of the Royal College of Surgeons by Mr. 
Stephen Paget. From some of the tumours buds have 
formed, and in one case the bud has grown into a 
minute branch. Every swelling on a tree, however, is 
not a woody tumour or xyloma; many are due to the 
irritation of insects. 
CANCERS.—We have now to consider the tumours 
whose main structural peculiarity is that they contain 
epithelium. The group is of great importance in that it 
includes the terrible disease known as cancer. It is only 
of late years that the term cancer has come to possess 
any strictly scientific significance. In the early days of 
pathological anatomy any tumour presenting malignant 
characters was termed cancer, but in the present day 
the term is restricted to tumours structurally resembling 
imperfectly formed glands. In order to appreciate the 
nature of cancer it will be advantageous for us to briefly 
study the evolution of glands in general. I can only 
attempt to give in abstract the large amount of evidence 
I have accumulated, in order to show that cancers are 
aberrant glandular formations, and may not inaptly be 
defined as “biological weeds.” 
In complex animals the free surface of the body and 
the alimentary canal is covered with cells differing from 
those found in the underlying tissues. Such cells are 
known collectively as epithelium, and though varying in 
shape in different situations and under various conditions, 
present identifying characters. This epithelium is prone 
