Geological Society of London. 121 
it is not a total replacement of one group by another that oc- 
curred ; both groups were represented at all times ; but as the 
one group approached a minimum in the development of spe- 
cific and generic types, the other approached a maximum, 
and vice versa. I think few geologists and naturalists who 
have studied both the paleozoic and the after—I must 
coin a word—neozoic mollusca will doubt that a large por- 
tion of the earlier Brachiopoda—the Productide, for example 
—performed the offices and occupied the places of the shal- 
Jower water ordinary bivalves of succeeding epochs. 
Now in this substitution, the replacement is not neces- 
sarily that of a lower group in the scale of organization by a 
higher. There is an appearance of such a law in many in- 
stances that has led over and over again to erroneous doc- 
trines about progression and development. The contrary — 
may be the case. Now that we have learned the true affini- 
ties that exist between the Bryozoa and the Brachiopoda, we 
can see in these instances the zoological replacement of a 
higher by a lower group, whilst, in the former view, equally 
true, of the replacement of the Brachiopoda by the Lamelli- 
branchiata, a higher group is substituted for a lower one. | 
Numerous cases might be cited of both categories. 
~ But can we not find something more in these replacements 
and interchanges than mere substitution, which is a pheno- 
menon manifested among minor and major groups within 
every extended epoch? Is there no law to be discovered in 
the grand general grouping of the substitutions that charac- 
terize the palzozoic epoch, when contrasted with all after- 
epochs considered as one, the Neozoic? It seems to me 
that there is, and that the relation between them is ore of 
. contrast and opposition ; in natural esmatt language is the 
relation of POLARITY. 
- The manifestation of this relation in organized nature is 
by contrasting developments in opposite directions. The 
well-known and often-cited instance of the opposition pro- 
gress of the vegetable and animal series, each starting from 
the same point—the point at which the animal and vegetable 
organisms are scarcely if at all distinguishable—may serve 
to illustrate the idea, and make it plain to those to whom the 
