CO-ORDINATED STRUCTURES 69 
Two blind boys examined by Spencer were both found 
to be able to distinguish with the tips of their fingers 
points separated by only ;; inch. And two skilled 
compositors could both distinguish in this way points 
placed no more than 4, inch apart, so that a person 
with a trained sense of touch acquires a considerably 
finer development of this faculty than an ordinary 
individual. 
If, then, acquired characters of this kind are in- 
herited, even to an extremely minute extent, such as 
would be scarcely perceptible in a single generation, 
the account of the origin of the observed phenomena 
would be complete. 
As a second argument, Herbert Spencer points out 
the difficulty of accounting for the development of 
co-ordinated sets of structures by the action of natural 
selection upon separate minute variations of the several 
parts concerned. 
The enormous horns of the ancient Irish elk, weigh- 
ing in some cases over a hundredweight, required 
specially strong neck muscles, bones, and ligaments, 
and strong fore legs for their support. But an increase 
in the strength of a single muscle following increased 
weight of the horns would be useless if unaccompanied 
by a corresponding increase in many other structures, 
and, if useless, could not be selected. The chance of 
all the parts concerned varying simultaneously in a 
corresponding direction is very small if these variations 
are really independent, and the chance of their doing 
so repeatedly is in such a case infinitesimal. 
Let us take another case of a similar nature. The 
