262 SECRETS OF ANIMAL LIFE 
one pair and two pairs. The evolution of a third 
pair of balancers would mean an increase in the 
functional problem of correlation without any 
corresponding advantage in the way of efficiency. 
John Burroughs writes somewhere about the gain 
it would be if we could open one pair of eyes after 
another and thus see more of the wonder of Nature. 
The probability is that we should see less, for the 
difficulty of correlating impressions would be insur- 
mountable. It is significant that those backboneless 
animals that have many eyes have little vizion, and 
that the unpaired, median, upward-looking, pineal 
eye which some backboned animals possessed has 
not been retained as an eye in higher types. It 
may be, then, that limits imposed by growth-neces- 
sities, by the persistence of well-defined structure- 
units, and by pre-existing organization may throw 
some additional light on the fact of convergence. 
