TRANSACTIONS OF SECTION D. 15 
Protozoan phase, we are led to believe that its evolution was probably very slow 
as compared with the rate which it subsequently attained. But this conclusion is 
of the utmost importance; for the history contained in the stratified rocks nowhere 
reveals to us the origin of a Phylum. And this is not mere negative evidence, but 
positive evidence of the most unmistakable character. All the five Ccelomate 
Phyla which occur fossil appear low down in the Paleozoic rocks, in the Silurian 
or Cambrian strata, and they are represented by forms which are very far from being 
primitive, or, if primitive, are persistent types, such as Chiton, which are now 
living. Thus Vertebrata are represented by fishes, both sharks and ganoids; the 
Appendiculata by cockroaches, scorpions, Limulids, Trilobites, and many Crustacea ; 
the Mollusca by Nautilus and numerous allied genera, by Dentalium, Chiton, 
Pteropods, and many Gastropods and Lamellibranchs ; the Gephyrea by very 
numerous Brachiopods, and many Polyzoa ; the Echinoderma by Crinoids, Cystoids, 
Blastoids, Asteroids, Ophiuroids, and Echinoids. It is just conceivable, although, 
as I believe, most improbable, that the Vertebrate Phylum originated at the time 
when the earliest known fossiliferous rocks were laid down. It must be remem- 
bered, however, that an enormous morphological interval separates the fishes which 
appear in the Silurian strata from the lower branches, grades, and classes of the 
Phylum in which Balanoglossus, the Ascidians, Amphioxus, and the Lampreys are 
placed. The earliest Vertebrates to appear are, in fact, very advanced members of 
the Phylum, and, from the point of view of anatomy, much nearer to man than tu 
Ampftioxus. If, however, we grant the improbable contention that so highly 
organised an animal as a shark could be evolved from the ancestral vertebrate in 
the period which intervened between the earliest Cambrian strata and the Upper 
Silurian, it is quite impossible to urge the same with regard to the other Phyla. 
It has been shown above that when these appear in the Cambrian and Silurian, 
they are flourishing in full force, while their numerous specialised forms are a 
positive proof of a long antecedent history within the limits of the Phylum. 
If, however, we assume for the moment that the Phyla began in the Cambrian, 
the geologist’s estimate must still be increased considerably, and perhaps doubled, 
in order to account for the evolution of the higher Phyla from forms as low as 
many which are now known upon the earth ; unless, indeed, it is supposed, against 
the weight of all such evidence as is available, that the evolutionary history in 
these early times was comparatively rapid. 
To recapitulate, if we represent the history of animal evolution by the form of 
a tree, we find that the following growth took place in some age antecedent to the 
earliest fossil records, before the establishment of the higher Phyla of the Animal, 
Kingdom. The main trunk representing the lower Protozoa divided, originating 
the higher Protozoa; the latter portion again divided, probably in a threefold man- 
ner, originating the two lowest Metazoan Phyla, constituting the Ccelentera. The 
branch representing the higher of these Phyla, the Nematophora. divided, origina- 
ting the lower Coelomate Phyla, which again branched and originated the higher 
Phyla. And, as has been shown above, the relatively ancestral line, at every 
stage of this complex history, after originating some higher line, itself continued 
down to the present day, throughout the whole series of fossiliferous rocks, with 
but little change in its general characters, and practically nothing in the way of 
progressive evolution. Evidences of marked advance are to be found alone in the 
most advanced groups of the latest highest products—the Phyla formed by the 
last of these divisions. 
It may be asked how is it possible for the zoologist to feel so confident 
as to the past history of the various animal groups. I have already explained 
that he does not feel this confidence as regards the details of the history, 
but as to its general jines. The evidence which leads to this conviction js 
based upon the fact that animal structure and mode of cevelopment can be, and 
have been, handed down from generation to generation from a period far more 
remote than that which is represented by the earliest fossils; that fundamental 
facts in structure and development may remain changeless amid endless changes of 
a more general character; that especially favourable conditions have preserved 
