236 EVOLUTION AND DISEASE. 
in the following particulars. The micro-organism or 
causative agent has not yet been isolated, and we have 
no satisfactory evidence that a sarcoma can be inoculated 
into another animal. Nevertheless the two forms of 
tumours agree in the general principle of structure, 
disastrous effects upon the life of the individual, and in 
a tendency to infect the system. Careful research will 
probably establish before very long a poison or micro- 
organism for each of the various types of sarcoma. My 
own inquiries into these tumours has long served to con- 
vince me that such will be the case, and a few of the 
reasons will be briefly detailed. Every day experience 
teaches that tumours in the human subject are extremely 
common. Attendance at a veterinary infirmary will 
soon convince a regular visitor that tumours are frequent 
in horses, cattle, sheep, and dogs. A long and careful 
personal attendance at many thousand /ost-mortem 
examinations of wild animals, dying in captivity, has 
disclosed the fact that such animals are rarely affected 
with tumours. A critical analysis of facts further shows 
that in man cancer is more common than infective 
tumours. In domesticated mammals cancer, in the 
sense in which it will be employed later, is unusual, 
whilst infective tumours are extremely common. In wild 
animals nearly all the tumours belong to the infective 
sranulomata, only a few cases of cancer being known. 
It may be useful to detail one or two typical specimens 
of sarcomata from animals. 
The first is a round-celled sarcoma growing in the 
subcutaneous tissues of the neck of a hen. It is of the 
size of a chestnut, and is surrounded by a capsule of 
