ATTEMPT TO DEDUCE VERTEBRATE EYES FROM THE SKIN. 367 
slow and uniform development of an organ, viz. the eye, 
through the lapse of time. Its ontogenetic development 
cannot possibly repeat the evolutionary process, because, when 
the eye was in the earliest stages, the embryo early acquired 
the habit of producing the organ in that primitive stage by a 
short cut. This short cut the embryo has never given up, and 
it has thus lost all chance of being able to repeat the subse- 
quent evolutionary processes. The development, therefore, of 
all these later specialisations must necessarily be almost purely 
adaptive. 
With regard to the pineal eye, somewhat the same explana- 
tion of its ontogeny is suggested. The rudimentary optic in- 
vaginations, situated close to the median line, early became 
involved in the developing medullary axis. They were, 
however, not adaptive structures, but the actual optic pits. 
Again, in this case also, the connection between the developing 
optic pit and the medullary axis or groove would be found in 
the optic nerve. The most serious objection to this suggestion 
is probably to be found in the fact that the epiphysis seems to 
appear later than the first rudiments of the Vertebrate eyes, 
whereas it ought, perhaps, to appear earlicr. As a matter of 
fact, very little reliance can be laid upon the order of appear- 
ance. The functional eyes, whether pineal or definitive, seem 
to be hurried on the scene almost before anything else. As 
soon as the pineal eye became functionless or of less importance 
than the definitive eyes, there would be no hurry in its develop- 
ment, and the definitive eyes might appear first, although 
historically much later developments than the pineal eyes. 
It may be objected that there is not the same difficulty in 
supposing the pineal eye to have developed from the brain, it 
being but a single vesicle with a simple nerve-stalk, so that 
its ontogeny might repeat its historical evolution. Spencer, 
who assumes the embryology of the eye to be in the main 
recapitulatory, compares the epiphysis with the tunicate eye, 
but does not believe that it began as an eye. In order 
to get over the difficulties in the way of deducing the pineal eye 
structurally from the tunicate eye, the possibility of which 
