Theory of Change of Function 
to use the words of Milnes Marshall, “a closed 
sac lying just underneath the vertebral column. 
In many fish it acquires a connection by a duct 
with some part of the alimentary canal. It then 
becomes an accessory breathing organ, especially 
in those fish which are capable of living out of 
water for a time, e.g. the Protopterus of America. 
An interesting series of modifications exists con- 
necting the air-bladder with the lung of the 
higher vertebrates, which is undoubtedly the 
same organ.” 
This theory, however, does not seem adequate 
to explain the origin of all organs. It does 
not explain, for example, how limbs developed 
in a limbless organism. Wallace tried to 
avoid the difficulty by asserting that it is un- 
reasonable to ask a new theory that it shall 
reveal to us exactly what took place in remote 
geological ages and how it took place. To this 
the obvious reply is, firstly, that we ought not 
to give unqualified acceptance to any theory of 
evolution until it does afford us such explana- 
tions, and, secondly, that the theory of the origin 
of species by means of natural selection is no 
longer a new one. 
Latterly, however, Wallace appears to have 
given up all hope of being able to account for 
the origin of new organs by means of natural 
selection, for he states on page 431 of the issue 
of the Fortnightly Review for March 1909: 
37 
