18 INTRODUCTION. 



shell-fish of various kinds, may easily excuse a popular error of 

 this nature ; but for the Scientific Naturalist of the present day 

 to give credence to it, would be unpardonable. 



The study of Natural History, then, has an obvious tendency, 

 not only to encourage the habit of correct and unprejudiced 

 observation in its votaries, but to call into exercise the discri- 

 minating powers, which shall teach them to attach their due 

 value to the statements of others. Upon the advantage of such 

 a kind of mental cultivation, it is unnecessary here to dwell. It 

 is useful in every situation, in every relation of life. It enables 

 us to suspend our judgment, when we are not satisfied of the 

 stability of the grounds upon which we are to decide ; and leads 

 us to draw the line between suspicious incredulity on the one 

 hand, and too ready assent to improbable and unconfirmed state- 

 ments on the other. In the cultivation of this habit, the study 

 of Natural History has an obvious advantage over that of the 

 more exact Sciences. We have not yet the same guidance 

 afforded by general laws, as that which they possess, and which 

 enables the physical philosopher to decide at once on the truth 

 of statements submitted to him. Tell a man acquainted with 

 the simple laws of Matter, that a perpetual motion had been 

 invented, depending upon strictly mechanical principles; and he 

 will not believe it, because he knows it to be a physical impos- 

 sibility. In Natural History we are to a certain extent equally 

 safe in forming a positive decision. The differences between the 

 skeleton of Man, and that of an Elephant or Lizard, are now 

 sufficiently well known, to prevent our giving credence to the 

 marvellous accounts of such prodigies, as a Man 25 feet long ; 

 these cease to astonish us (except as regards the ignorance that 

 propagated them), now that we are acquainted with their true 

 explanation. Nor are we in danger of allowing ourselves to be 

 led away by an English writer of much learning, who, so late as 

 the middle of the 17th century, attributed the origin of fossil 

 shells and fishes to " a plastic virtue latent in the earth." But 



