14 
. POPULAR SCIENCE REYIEW. 
will be struck with astonishment when he discovers, on further 
examination, that an extraordinary variety of structural arrange- 
ments obtains in the descending scale of animals. In compli- 
city and minuteness of detail the vertebrate eye surpasses all 
others. Yet in so far as optical laws postulate a certain ar- 
rangement of the eye structures it might be said to explain 
itself : that is to sa}^, the actually existing disposition of trans- 
parent refracting media is just such as might be imagined a 
priori as a consequence of known laws of light transmission, 
and known properties of matter. With such conceptions of plan 
and principle in his mind, the observer cannot but feel per- 
plexed with the apparent contradictions which he meets with, 
when he examines the structure of the eyes of the lower in- 
vertebrata. The seemingly simple plan of the mammal eye 
loses itself in a wondrous diversity of external form and 
internal structure. In one case, elements considered to be 
essential are apparently missing ; in another, additions are 
found which have no counterpart in eyes supposed to be more 
perfect. And a comparison of extreme instances would lead 
to the inference of a total irreconcilability of constructive 
plan, and even to the suspicion that the function can scarcely 
be one and the same in each case, were it not that organs of an 
intermediate character are found in the several classes of 
animals, which supply connecting links, and enable us to trace a 
continuity of really essential parts, and a constancy of funda- 
mental conditions, throughout the whole series. The final 
result of this comparative method of study is highly interesting 
and important : namely, that the same laws which have been 
found to obtain in the rationale of human vision, equally apply 
to every creature possessing a faculty of sight, which amounts 
to perception of form and colour. Just as the observed pertur- 
bations of the calculated movements of distant planets, instead 
of upsetting the doctrine of an universal law of gravity, trium- 
phantly confirm it, by the discovery of new heavenly bodies, 
new elements of calculation by which the previous errors receive 
due correction, so do the unexpected and startling contrarieties of 
- structural arrangement in the eyes of different animals eventu- 
ally lead to the fuller proof of principles seemingly jeopardised 
by unsuspected complications. So again the inseparable con- 
nection between every known kind of eye and the phenomena 
of light transmission (and such plienomena are as universal and 
fundamental as those of gravitation) compels us to receive with 
implicit confidence the surprising evidences of this mutual re- 
lation of the eye to light, and light to the eye, incidentally af- 
f-.rdcfl by the discovery of fossilised corneal structures of marine 
animals living in remote ages, but now extinct. Thus a micro- 
bCO[>ic fragment of rock may reveal to us facts respecting the 
