230 EVOLUTION AND DISEASE. 
The bodies of animals contain cavities unprovided 
with ducts : occasionally fluid exudes into such cavities 
and dilates them into tumours. Such are termed exuda- : 
tion cysts. These have little interest for us. . 
The bodies of complex animals contain many ducts 
and passages which were presumably functional in their — 
remote ancestors, but are functionless in existing forms. 
Such are collectively known as obsolete canals. These 
canals are occasionally distended with fluid and form — 
tumours often of large size. This group is of great im- 
portance to us, as the germs of such tumours are due to— 
modifications induced in animal organization mainly by 
change in the environment, and inheritance of structural 
variations. 
A very striking example occurs in connection with the 
ovary. In many mammals and reptiles a collection of 
vestigial tubular structures has been observed in connec- 
tion with this organ. In the higher mammals they con- 
sist of a series of short vertical tubules dipping into the 
hilum of the ovary ; above, they end blindly in a duct. 
This vestigial tubular organ is known as the parovarium, 
and in the male it forms an important part of the excre- 
tory apparatus of the reproductive organs. 
From some cause quite unknown to us, one or other of 
these tubules may become distended into tumours hold- 
ing many ounces, or even pints, of fluid. Such tumours ~ 
may jeopardise life from mechanical reasons or induce 
death by reason of secondary changes in their interior. 
Cysts of this kind are met with in mammals of all kinds, 
in birds, reptiles, and in amphibians. 
Hen birds occasionally furnish an admirable example 
