TRANSACTIONS OF SECTION D. 821 
below the horizon, which for the purpose of this address I am considering as the 
lowest fossiliferous stratum. We are looking forward to the new light which will 
be thrown upon this form in the communication of its veteran defender, Sir 
William Dawson, whom we are all glad to welcome. 
Passing the Radiolaria, with delicate skeletons less suited for fossilisation, and 
largely pelagic, and therefore less likely to reach the strata laid down along the 
fringes of the continental areas, the next Phylum which is found in a fossil state 
is that of the Porifera, including the sponges, and divided into two classes, the 
Calcispongie and Silicospongize. Although the fossilisation of sponges is in many 
cases very incomplete, distinctly recognisable traces can be made out in a large 
number of strata. From these we know that representatives of all the groups of 
both classes (except the Halisarcide, which have no hard parts) occurred in the 
Silurian, Devonian, and Carboniferous systems, The whole Phylum isan example of 
long persistence with extremely little change. And the same is true of the Nema- 
tophora: new groups indeed come in, sometimes extremely rich in species, such as 
the Palzozoic Rugose corals and Graptolites; but they existed side by side with 
representatives of existing groups, and they are not in themselves primitive or 
ancestral. A study of the immensely numerous fossil corals reveals no advance in 
organisation, while researches into the structure of existing Aleyonaria and Hydro- 
corallina have led to the interpretation of certain Palzeozoic forms which were pre- 
viously obscure, and the conclusion that they find their place close beside the 
living species, 
All available evidence points to the extreme slowness of progressive evolu- 
tionary changes in the Ceelenterate Phyla, although the Protozoa, if we may judge 
by the Reticularia (Foraminifera), are even more conservative. 
When we consider later on the five Coelomate Phyla which occur fossil, we 
shall find that the progressive changes were slower and indeed hardly appreciable 
in the two lower and less complex Phyla, viz., the Echinoderma, and Gephyrea, 
as compared with the Mollusca, Appendiculata, and Vertebrata. 
Within these latter Phyla we have evidence for the evolution of higher groups 
presenting a more or less marked advance in organisation. And not only is the 
rate of development more rapid in the highest Phyla of the animal kingdom, but 
it appears to be most rapid when dealing with the highest animal tissue, the 
central nervous system. The chief, and doubtless the most significant, difference 
between the early Tertiary mammals and those which succeeded them, between the 
Secondary and Tertiary reptiles, between man and the mammals most nearly 
allied to him, is a difference in thesize of the brain. In all these cases an enormous 
increase in this, the dominant tissue of the body, has taken place in a time which, 
geologically speaking, is very brief. 
hen glancing later on over the evolution which has taken place within 
the Phyla, further details upon this subject will be given, although in this as in 
other cases the time at our disposal demands that the exposition of evidence 
must largely yield to an exposition of the conclusions which follow from its 
study. And undoubtedly a study of all the available evidence points to the con- 
clusion that in the lower grade, sub-grades, and Phyla of the animal kingdom 
evolution has been extremely slow as compared with that in the higher. We do 
not know the reason. It may be that this remarkable persistence through the 
stratified series of deposits is due to an innate fixity of constitution which has 
rigidly limited the power of variation; or, more probably perhaps, that the lower 
members of the animal kingdom were, as they are now, more closely confined to 
particular environments, with particular sets of conditions, with which they had to 
cope, and, this being successfully accomplished, natural selection has done little 
more than keep up a standard of organisation which was sufficient for their needs ; 
while the higher and more aggressive forms, ranging over many environments and 
always prone to encounter new sets of conditions, were compelled to undergo respon- 
sive changes or to succumb. But, whatever be the cause, the fact remains, and is of 
importance for our argument. When the ancestor of one of the higher Phyla 
was associated with the lower Phyla of the Ccelomate sub-grade, when further 
back it passed through a Ccelenterate, a higher Protozoan, and finally a lower 
