166 THE INDUCTIONS OF BIOLOGY. 



arms and legs do, when needful, fulfil, to some extent, each 

 others' offices. JSfot only in childhood and old age are the 

 arms used for purposes of support, but on occasions of emerg- 

 ency, as when mountaineering, they are so used by men in full 

 vigour. And that legs are to a considerable degree capable 

 of performing the duties of arms, is proved by the great 

 amount of manipulatory skill reached by them when the 

 arms are absent. Among the perceptions, too, there are ex- 

 amples of partial substitution. The deaf Dr Kitto described 

 himself as having become excessively sensitive to vibrations 

 propagated through the body ; and as so having gained the 

 power of perceiving, through his general sensations, those 

 neighbouring concussions of which the ears ordinarily give 

 notice. Blind people make hearing perform, in part, the 

 office of vision. Instead of identifying the positions and 

 sizes of neighbouring objects by the reflection of light from 

 their surfaces, they do this in a rude way by the reflection 

 of sound from their surfaces. 



We see, as we might expect to see, that this power of per- 

 forming more general functions, is great in proportion as 

 the parts have been but little adapted to their special func- 

 tions. In the hydra, where complete transposition of functions 

 is possible, the histological differentiation that has been estab- 

 lished, is extremely slight, or even inappreciable. Those parts 

 of plants which show so considerable a power of discharging 

 each others' offices, are not widely unlike in their minute 

 structures. And the tissues that in animals are to some 

 extent mutually vicarious, are tissues in which the original 

 cellular composition is still conspicuous. But we do not find 

 evidence that the muscular, nervous, or osseous tissues are 

 able in any degree to perform those processes which the 

 less differentiated tissues perform. Nor have we any 

 proof that nerve can partially fulfil the duty of muscle, 

 or muscle that of nerve. We must say, therefore, that 

 the ability to resume the primordial community of function, 



