286 THE ZOOLOGIST. 
even M. Jourdain, for while that immortal bourgeois had spoken 
prose for forty years without knowing it, the zoologist has for half 
a century called vertebrate an animal with no vertebre at all. 
This reconsideration of known facts has resulted in some, 
and in time, I doubt not, they will be followed by all, systematic 
zoologists, calling that great aggregation of forms to which belong 
Man, the Hawk, the Snake, and the Lancelet that of the Chordata, 
instead of that of the Vertebrata. 
It was about this time (in 1868) that Prof. Huxley enun- 
ciated certain principles which are of very great importance 
in Systematic Zoology; and they are to be found in one of 
the volumes of ‘The Ibis,’ which was at that time under the 
editorship of Prof. Newton, and in a letter in which Prof. Huxley 
defends himself against the criticism of that accomplished 
ornithologist. 
The part of the letter to which I wish you to pay attention 
runs thus :— 
“ Further, it must be recollected that the diagnosis of a group 
may rest not merely on a particular character confined to the 
group, but on a peculiar combination of characters. 
“ And it may happen that a well-defined group shall not have 
a single structural feature peculiar to itself, its peculiarity lying 
entirely in the mode of combination of those features.” 
You will ask, perhaps, where is the combination of characters 
on which Prof. Huxley insisted; you will say that, the presence 
of a notochord being a necessity for a dorsal nerve-cord, the 
fact of the co-existence of these two organs is not a matter of 
much importance. I need not discuss this objection with you, 
for I am willing, at least, to pass it by; for this is yet another 
organ—that, namely of respiration—which is formed in just the 
same way in Vertebrates and Ascidians. In all the Vertebrata 
we find that there appear at the sides slits or clefts formed by the 
inpushing of the layer of cells on the outer surface of the body ; 
these slits are met by outgrowths of the layer which lines some 
of the anterior portion of the digestive tract; the outgrowths and 
ingrowths meeting fuse, and give rise to a passage from the tract 
to the exterior. In fishes and in some—or for part of their 
lives all—Amphibians these slits serve as a means of passage 
for the water of respiration, which, entering by the mouth, 
bathes the slits, in the walls of which blood-vessels are well 
