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 

 i probably establish before very long a poison or micro- 

 - 1 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 post-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 

 granulomata, 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 



