238 EVOLUTION AND DISEASE. 
tumours in fish, frogs, birds, snakes, marsupials, rodents, 
carnivora, quadrumana, and ruminants. 
When sarcomata grow from bone, especially from 
the interior of a bone, they usually possess large num- 
bers of giant-cells. When originating in pigmented 
spots such as the black or pigment coat of the eye, or 
the pigment layer of the skin, they are of a deep black 
colour, and named in consequence melanotic. Grey 
horses are especially liable to this form of tumour, yet 
we have no reason to believe that the coloured races of 
mankind are more or even so prone to them as 
Europeans. 
Sarcomata do not always remain localized in this 
way. After the tumour has been growing for a time, 
other nodules make their appearance in different parts 
of the body, and not infrequently the secondary forma- 
tions are larger than the original tumour. These facts, 
and the general effects of such tumours, would alone 
cause us to suspect some parasitic agent, and what is of 
utmost importance, the early removal of the primary 
tumour occasionally prevents general infection. 
In the chapter on Inflammation, the relation of the 
leucocytes to bacterial invasion was described. Let us 
ascertain how it will elucidate the nature of sarcomata. 
The classification of these tumours is founded on the 
structural characters displayed by thin sections of the 
dead tumours under the microscope; they are then 
described as round-cell tumours, spindle-celled, melanotic 
or giant-celled. The appearance of a section of a round- 
celled sarcoma is exhibited in fig. 121. 
When fluid portions of such tumours are examined, 
