126 THE THEORY OF MUTATION 
kinds of carriages, ‘namely, omnibuses, hansoms, 
and four-wheelers, are specific and excellent illustra- 
tions of what I wish to express by mechanical types 
as distinguished from subtypes. Attempted im- 
provements in each of them are yearly seen, but none 
have as yet superseded the old familiar patterns, 
which cannot, as it thus far appears, be changed with 
advantage, taking the circumstances of London as 
they are. Yet there have been numerous subsidiary 
and patented contrivances, each a distinct step in 
the improvement of one or other of the three primary 
B A Cc B 18) Cc 
FIG. Io. 
types, and there are or may be an indefinite number of 
varieties in details, too unimportant to be subjects of 
patent rights.’* 
More recently Galton might have pointed out the 
introduction of motor traffic as illustrating a distinct 
mutation. 
The distinction between primary and subordinate 
positions of stability is further excellently illustrated 
by the model which is here represented, and which 
is known as Galton’s polygon (Fig. 10). 
* ‘Natural Inheritance,’ p. 26. 
