122 Anniversary Address to the 
use of the term POLARITY in geological science may not be 
familiar. In that case we speak of two groups being in the 
relation of polarity to each other, when the rudimentary 
forms of each are proximate, and their completer manifes- 
tations far apart. This relation is not to be confounded with 
- divergence, nor with antagonism. 
If we take the scale of geological formations, representing 
the succession of the leading divisions of time, and note for 
each of the epochs the known generic types present during 
its duration, we shall find there is not an equality of produc- 
tion, so to speak, at all times of fresh generic ideas. Genera 
have appeared, as it were, in batches. Iam forced to use 
expressions that seem almost irreverent, and a phraseology 
of a loose and popular kind, in order to convey the more vi- 
vidly my meaning. To talk of the appearance of a genus, 
that is, the appearance of an ideal type, is loose language, I 
am aware, but its meaning or intention can scarcely be misun- 
derstood. In the individuals of a species only can we have 
the embodiment of a generic idea; but in discussing 
a question of the kind I am considering, it is conve- 
nient to use the word genus as if it were a realized unit 
and an entity. We speak, as it were, through a diagram. 
Now if commencing, upon our scale, at the dawn of the pa- 
lxozoic epoch, and noting the beginning of genera or groups 
from the first known fauna up to the advent of man at the 
termination of the so-called tertiary epoch, we cannot fail to 
perceive the following general facts :— 
1. During the earlier and middle stages of the paleozoic 
epoch, there was a great development of generic ideas. 
2. During the middle and later stages of the neozoic epoch 
there was a great development of generic ideas. 
3. During the terminating stages of the palzeozoic epoch, 
the origination of generic ideas was very scantily mani- 
fested. 
4, During the commencing stages of the neozoic epoch, 
the origination of generic ideas was very scantily mani- 
fested. 
5. The majority of generic ideas that originated during 
the palewozoic epoch belong to groups (of various degrees of — 
