XXX 



Other species, more flexible and more obedient to extra- 

 neous impulses, have, for this very reason, reached the ut- 

 most limits which Nature had assigned to their variation, 

 under the operation of those circumstances to which they 

 were subjected within the area of the same geographical 

 region; and therefore, when submitted to climatic condi- 

 tions still more different, necessarily became extinct. 



Accordingly the humblest and simplest species of plants 

 may be at once the most durable and the most widely dif- 

 fused; just as in the animal kingdom certain shells, such 

 as the Lingnla, which existed during the earliest Silurian 

 epoch are said by some geologists to be found in a living 

 state at the present time, and thus to have survived all the 

 catastrophes and changes of condition which the earth has 

 undergone. 



But as Bishop Berkeley, from expounding the virtues of 

 tar-water, was led on to those high speculations respecting 

 the Anima Mundi and the mysteries of the Trinity, with 

 which his treatise concludes; so in this Preface I have 

 gradually rambled from the subject of the Essay which it 

 was my sole aim to introduce to the Public, until I came 

 upon the discussion of questions of an intricate nature, con- 

 cerning the origin of organic beings in general, and the 



