Notices of Memoirs—Prof. Boyd Dawkins’s Address. 468 
The distribution of the extinct orders in the animal kingdom, 
taken along with their distribution in the rocks, proves further that 
some types have varied more than others, and at various places in 
the geological record. In the Protozoa, Porifera, and Vermes there 
are no extinct orders; among the Coelenterates one, the Rugosa; in 
the Echinodermata three, Cystideans, Hdriasterida, and Blastoidea ; 
in the Arthropoda two, the Trilobita and Hurypterida. All these, 
with the solitary exception of the obscure order Rugosa, are found 
only in the Primary rocks. Among the Pisces there are none; in 
the Amphibia one, the Labyrinthodonts ranging from the Car- 
boniferous to the Triassic age. Among the Reptilia there are at 
least six of Secondary age, Plesiosauria, Ichthyosauria, Dicynodontia, 
Pterosauria, Theriodontia, Dinosauria ; in the Aves two, the Saururee 
and Odontornithes, also Secondary. In the Mammalia the Amblypoda, 
Tillodontia, Condylarthra, and Toxodontia represent the extinct 
orders—the first three Early Tertiary, and the last Pleistocene. It 
is clear, therefore, that while the maximum amount of ordinal 
variation is presented by the Secondary Reptilia and Aves, all the 
extinct orders in the Tertiary are Mammalian. 
If we turn from the extinct orders to the extinct species, it will 
also be found that the maximum amount of variation is presented 
by the plants, and all the animals, excepting the Mammalia, in the 
Primary and Secondary periods. 
The general impression left upon my mind by these facts is that, 
while all the rest of the animal kingdom had ceased to present 
important modifications at the close of the Secondary period, the 
Mammalia, which presented no great changes in the Secondary rocks, 
were, to quote a happy phrase of Professor Gaudry, “en pleine 
évolution” in the Tertiary age. And when, further, the singular 
perfection of the record allows us to trace the successive and gradual 
modifications of the Mammalian types from the Eocene to the close 
of the Pleistocene Age, it is obvious that they can be used to mark 
subdivisions of the Tertiary Period, in the same way as the reigns 
of kings are used to mark periods in human history. In my opinion 
they mark the geological horizon with greater precision than the 
remains of the lower members of the animal kingdom, and in cases 
such as that of Pikermi, where typical Miocene forms, such as 
Dinotheria, are found in a stratum above an assemblage of marine 
shells of Pliocene age, it seems to me that the Mammalia are of 
greater value in classification than the Mollusca, some of the species 
of which have been living from the Eocene down to the present day. 
Yet another important principle must be noted. The fossils are 
to be viewed in relation to those forms now living in their respec- 
tive geographical regions. The depths of the ocean have been where 
they are now since the earliest geological times, although continual 
geographical changes have been going on at their margins. In 
other words, geographical provinces must have existed even in the 
earlier geological periods, although there is reason to believe that 
they did not differ so much from each other as at the present day. 
It follows from this that the only just standard for comparison in 
