TRANSACTIONS OF SECTION I. 971 
creatures. When we examine these ancient fishes we find such forms as Pteraspis, 
Pterichthys, Astrolepis, Bothriolepis, Cephalaspis, all characterised by the enormous 
disproportion between the extent of the head region and that of the body. Such 
forms would have but small power of locomotion, and further evolution consisted 
in gaining greater rapidity and freedom of movements by the elongation of the 
abdominal and tail regions, with the result that the head region became less and 
less prominent, until finally the ordinary fish-like form was evolved, in which the 
head and gills represent the original head and branchial chamber, and the flexible 
body, with its lateral line nerve and intestine innervated by the vagus nerve, 
represents the original small tail-like body of such a form as Pterichthys. 
Nay, more, the very form of Pterichthys and the nature of its two large oar-like- 
appendages, which, according to Traquair, are hollow, like the legs of insects, sug- 
gest a form like Eurypterus, in which the remaining locomotor appendages had 
shrunk to tentacles, as in Ammoccetes, while the large oar-like appendages still 
remained, coming out between the upper and lower lips and assisting locomotion. 
The Ammoccetes-like forms which in all probability existed between the time of 
Eurypterus and the time of Pterichthys have not yet been found, owing possibly 
to the absence of chitin and of bone in these transition forms, unless we may 
count among them the recent find by Traquair of Paleeospondylus Gunni. 
The evidence of paleontology, as far as it goes, confirms absolutely the evi- 
dence of anatomy, physiology, phylogeny, and embryology, and assists in forming 
a perfectly consistent and harmonious account of the origin of vertebrates, the 
whole evidence showing how Nature made a great mistake, how excellently she 
rectified it, and thereby formed the new and mighty kingdom of the Vertebrata. 
Consideration of Rival Theories. 
Tn conclusion I would ask, What are the alternative theories of the origin of 
vertebrates? It is a strange and striking fact how often, when a comparative 
anatomist studies a particular invertebrate group, he is sure to find the vertebrate 
at the end of it: it matters not whether it is the Nemertines, the Capitellide, 
Balanoglossus, the Helminths, Annelids, or Echinoderms ; the ancestor of the verte- 
brate is bound to be in that particular group. Verily I believe the Mollusca alone 
have not yet found a champion. On the whole I imagine that two views are 
most prominent at the present day—(1) to derive vertebrates from a group of 
animals in which the alimentary canal has always been ventral to the nervous 
system; and (2) to derive vertebrates from the appendiculate group of animals, 
especially annelids, by the supposition that the dorsal gut of the latter has become 
the ventral gut of the former by reversion of surfaces. Upon this latter theory, 
whether it is Dohrn or van Beneden or Patten who attempts to homologise similar 
parts, it is highly amusing to see the hopeless confusion into which they one and 
all get, and the extraordinary hypotheses put forward to explain the fact that the 
gut no longer pierces the brain. One favourite method is to cut off the most 
important part of the animal, viz. his supra-cesophageal ganglia, then let the mouth 
open at the anterior end of the body, turn the animal over, so that the gut is now 
ventral, and let a new brain, with new eyes, new olfactory organs, grow forward 
from the infra-cesophageal ganglia. Another ingenious method is to separate the 
two supra-cesophageal ganglia, let the mouth tube sling round through the separated 
ganglia from ventral to dorsal side, then join up the ganglia and reverse the 
animal, The old attempts of Owen and Dohrn to pierce the dorsal part of the 
brain with the gut tube either in the region of the pineal eye or of the fourth 
ventricle have been given up as hopeless. Still the annelid theory, with its 
reversal of surfaces, lingers on, even though the fact of the median pineal eye is. 
sufficient alone to show its absolute worthlessness. 
Then, as to the other view, what a demand does that make upon our credulity! 
We are to suppose that a whole series of animals has existed on the earth, the 
development of which has run parallel with that of the great group of appendiculate 
animals, but throughout the group the nervous system has always been dorsal to 
the alimentary canal. Of this great group no trace remains, either alive at the 
3R2 
