272 T. Davidson — Wkat is a Brachiopod ? 
is no modification, that I can see, of any of the forras that con- 
tinue beyond, as far as the Brachiopoda appear to be concerned, 
and why should a nuraber of genera, such as Lingula, Discina, Crania, 
and Rhynchonella, have continued to be represented with the same 
characters and often with but small modification in shape during the 
entire sequence of geological strata ? Why did they not offer 
modifications or alter during those incalculable ages ? Limiting 
myself to the Brachiopoda, let us see what further they will teil us 
upon this question. Taking the present state of our knowledge as 
a guide, but admitting, at the same time, that any day our conclusions 
and inductions may require to be modified by fresh discoveries, let 
us ascertain whether they reveal anything to Support Darwinian 
ideas. We find that the larger number of genera made their first 
appearance during the Palaeozoic periods, and since they have been 
decreasing in number to the present period. We will leave out of 
question the species, for they vary so little that it is often very 
difficult to trace really good distinctive characters between them ; it 
is different with the genera, as they are, or should be, founded on 
much greater and more permanent distinctions. Thus, for example, 
the family Spiriferidce includes genera which are all charac- 
terized by a calcified spiral lamina for the support of the brachial 
appendages ; and however varied tliese may be, they always retain 
the distinctive characters of the group from their first appearance to 
their extinction. The Brachiopodist labours under the difficulties of 
not being able to determine what are the simplest, or which are the 
highest families into which either of the two great groups of his 
favourite dass is divided ; so far then he is unable to point out any 
evidence favouring progressive development in it. But, confining 
himself to species, he sees often before him great varietal changes, so 
much so, as to make it difficult for him to define the species ; and it 
leads him to the belief that such groups were not of independent 
origin, as was universally thought before Darwin published his great 
work on the Origin of Species. But in this respect the Brachiopoda 
reveal nothing more than other groups of the organic kingdoms. 
It would appear that the earliest forms among the Brachiopoda 
are referable to the division Tretenterata, which includes the genera 
Lingulella, Lingula, Discina, and Obolella. Of tliese only Lingula 
and Discina have lived on with but slight modifications in external 
shape during the entire sequence of geological time ; and they are 
still represented by several species. But in rocks somewhat later 
in age (from the middle beds of the Menevian group or Lower Lin- 
gula flags) to those in which the above genera are found, there occurs 
a species of Qrtlns (0. ITicksii), which may possibly be the first re- 
presentative, as far as we are aware, of the division C listenter ata. 
On this point, however, I would refer to Professor King’s note pre- 
viously given. Since the Cambrian period both divisions continue 
to be represented without apparently showing a tendency to pass one 
into the other. Now although certain genera, such as Terebratula, 
Rhynchonella, Crania, and Discina, have enjoyed a vei-y considerable 
geological existence, there are genera, such as Stringocephalus, Uncites, 
