ON THE LIMITS OF THE CLASS OF FISHES. 
BY THEODORE GILL, M.D., PH.D. 
In the classification of the animal kingdom the vague ideas 
prevalent among the vulgar and originating in prejudices based on 
habitat or external appearance have been more or less reflected, 
and special forms associated with earth, air and water. 
were the inhabitants of the “ element” associated together on the 
one hand, and separated from those of other “ elements” in the 
other by the ancient cosmologists and poets.* This was the first 
generalization or attempt to combine the groups which from all 
time have been recognized as “beasts” or “ animals,” “ birds,” 
“ fishes,” and others in still more comprehensive groups. But, 
in time, and as investigation was directed to the structure of 
animals, it was found that the preconceived ideas respecting the 
relations of the various forms to the media which they inhabited, 
were by no means the correct expressions of the relations of such 
forms in structural features. The recognition of this fact resulted 
in the admission of several classes (anticipating the definite ideas 
which are now associated with such groups) and the virtual sepa- 
ration of the vertebrates from the invertebrates. But the ancient 
* In the beginning God created the k and th th. And the earth was without 
form and void. 
the fi nt wad divided th ters tl fi t 
pact tb \cka Ga geet God said: Let t 
waters under the heaven be gathered ae unto one place, aA let the dry je 
appear: arn 
said: Le tke waters, bring phe pigs ged the moving aibei that hath 
life, and pan that may afe en firmament « of — 
nd God said: Let the earth bring forth the d, cattle and 
creeping thing, ces beast a the earth after his kind: and it was so.— Genesis, sx A 
mees L2 T, ig 20, 2 
fr th 
x d by several of the ancient poets and especially by Ovid in 
the following lines :— 
mare et tellus, et, quod tegit omnia, celum, 
Bans erat toto nature vultus in orbe, 
Quem dixere Chaos; rudis indigestaque mo, 
Hane Deus et melior litem ‘natura diremit : : 
ET ~~ terras, et a oe — ; 
iscibue 
erra feras cepit; volucres agitabilis aër. 
Orie, ernie I,l.5- -7, 21- 2, 72- T5. 
H ee 
