CONVERGENCE IN EVOLUTION = 259 
type of structure, we can theoretically suppose 
that it will be reached in the course of time by the 
selection of variations, along two or more distinct 
routes. In some cases, however, it is an assumption 
to say that the type of structure attained is the 
only possible solution of the problem, or by far 
the best and easiest. Moreover, the difficulty of 
Darwinian interpretation increases a little when the 
mode of individual development is quite different in 
the two cases, and when the similarities include 
minute details. The striking superficial resemblance 
between burrowing amphibian, burrowing lizard, 
and burrowing snake—a familiar puzzle to students 
of elementary zoology—is readily intelligible, for 
the worm-like shape is the. only one possible; but 
is there not more difficulty in accounting for the 
telescope eyes of unrelated fishes and cuttlefishes, 
where the general idea is the same, though there 
are marked differences in deail and in development? 
Some thoughtful students of evolution* see no 
special riddle in the attainment of closely similar 
adaptations by unrelated types. But as we do not 
regard with much satisfaction or confidence the 
available biological interpretations of the way in 
which creatures of very diverse flesh and blood 
have often worked out the same solution to 
a problem, we would make three suggestions. 
(1) There are probably architectural laws of growth 
and differentiation of which we have not more than 
*See for a clear-headed discussion a letter by Mr. George 
Hookham in The New Statesman, 6th Jan., 1917, p. 325. 
