44 EVOLUTION AND DISEASE. 
by no means harmless, for occasionally it enlarges and 
becomes occupied by tumours, sometimes of large size 
and complex constitution, which cause death from 
mechanical interference with the brain. 
Darwin has pointed out that rudimentary (vestigial) 
parts are apt to be highly variable. This variability 
he thought was largely due to uselessness, and there- 
Fic. 22.—A magnified view of a section through the pineal 
eye of Hatteria. (After Baldwin Spencer.) 
fore to natural selection having no power to check — 
deviations in the structure of such parts. 
It will be interesting to study this view of the matter 
in connection with oviducts, which are of frequent occur-_ 
rence as anomalies in the males of unisexual animals, 
are 
especially as two reasons can be advanced for their 
vagarious conduct. In the first place, I shall endea- 
vour to, show that they have undergone a remarkable 
