h 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



able, — may serve to illustrate the idea, and make it plain to those 

 to whom 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 manifestations far apart. This re- 

 lation is not to be confounded with divergence, nor with antagonism. 

 If we take the scale of geological formations, representing the suc- 

 cession 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 production, so to speak, at all times 

 of fresh generic ideas. Genera have appeared, as it were, in batches. 

 I am forced to use expressions that seem almost irreverent, and a 

 phraseology of a loose and popular kind, in order to convey the more 

 vividly 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 misunderstood. In the in- 

 dividuals 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 

 convenient 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 com- 

 mencing, upon our scale, at the dawn of the palaeozoic 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 palaeozoic 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 palaeozoic epoch the ori- 

 gination of generic ideas was very scantily manifested. 



4. During the commencing stages of the neozoic epoch the origi- 

 nation of generic ideas was very scantily manifested. 



5. The majority of generic ideas that originated during the pa- 

 laeozoic epoch belong to groups (of various degrees of generic inten- 

 sity) which are characteristically palaeozoic, i. e. have their maximum 

 development and variety during the palaeozoic epoch, or else are even 

 exclusively palaeozoic. 



6. The majority of generic ideas that originated during the neozoic 

 epoch belong to groups which are characteristically neozoic in the 

 same manner. 



7. The minimum development of generic ideas in time is at or 

 about the passage or point of junction of the palaeozoic and neozoic 

 epochs. 



8. Groups characteristically palaeozoic swell out, as it were, in a 

 direction foicards, not from, the commencement of the palaeozoic 

 epoch. 



9. Groups characteristically neozoic swell out in a direction fro)n 

 the commencement of the neozoic epoch. 



That there are apparent exceptions to these general facts I do not 

 pretend to deny, but the rules are so much more powerful than the 



