xxxvi INTRODUCTION. 



organic remains which we discover are always those only of 

 the simpler forms, but chiefly the Mollusca, whose calcareous 

 coverings have remained after the softer parts have decayed. 

 At a long posterior era, we find the remains of Fishes, the 

 next in order of the animal tribes above the Mollusca ; and, 

 at length, as successive periods rolled on, we find the remains 

 of Reptiles, and at length of Birds and Mammalia, all con- 

 forming to the more general types of animal forms, but all 

 distinct as species from any now inhabiting the land or wa- 

 ters of the globe : and continually, as the earth approaches 

 to the present conditions of its surface, new species appear, 

 until at last we discover animals identical with those now 

 existing, or differing slightly from them. 



By Species we designate animals resembling one another 

 in their essential characters, and possessed of the p'ower, 

 common to the vegetable and animal kingdoms, of reproduc- 

 ing individuals similar to themselves and to one another. 

 Now, in the past eras, as in the present, we find animals 

 essentially alike, and which we infer were possessed of the 

 power of reproducing the like forms. A question which 

 enters into the fair range of philosophical inquiry may arise, 

 whether, in the course of immense periods of time, these 

 species have been so modified, in obedience to some grand 

 system of natural laws, as to become suited to new conditions 

 of external nature, or whether each mutation has been a new 

 act of creative power, called forth as the occasion arose, to 

 produce a new race of beings 1 We cannot certainly resolve 

 this problem by any knowledge we possess of the actual 

 changes of animal species ; and it is only from analogy that 

 we can venture to infer, that the operation of the same laws, 

 under which species have been called forth by the decrees of 

 an Omnipotent Pow r er, may have adapted species to new 

 states of existence. Animals, it may be believed, must be 

 suited to the conditions of external nature under which they 

 are called to exist. The digestive organs must be adapted 

 to the nature of the aliment from which the system of the 



