66 THE THEORY OF NATURAL SELECTION 
ance. Since all arguments in favour of the evolution 
of species were incomplete unless some means by 
which such an evolution could take place had been 
suggested, Spencer adopted the Lamarckian theory of 
modification, and to this he always firmly adhered, 
though admitting the validity of natural selection as 
an additional factor in the process. Some of Spencer’s 
arguments in favour of a belief in the inheritance of 
acquired characters are well worth repetition, since 
they have never been altogether refuted. 
Herbert Spencer’s argument consisted mainly in the 
enumeration of structures the origin of which cannot 
be explained by natural selection. On the other hand, 
the inheritance of acquired characters, if this form of 
inheritance could be proved to have a real existence 
—as Spencer believed it could—was shown to be a 
perfectly adequate explanation of the origin of the 
structures in question. In 1893, when Spencer up- 
held his opinion for the last time, Bateson had not yet 
pointed out that the facts of definite and discontinuous 
variation afford an alternative way out of some of 
these difficulties. In the absence, therefore, of any 
other effective cause, the result of the argument 
pointed strongly to the conclusion that the inheritance 
of acquired characters must be a reality. 
The first of Spencer’s arguments was based upon 
the different powers of tactua] discrimination which 
are to be found in different parts of the human body- 
The degree of this sensitiveness may De estimated by 
the use of a pair of compasses, the points of which can 
be set at different distances apart. It is then found 
