L] INTRODUCTION. XVli 



" Coralline Crag ") are precisely similar in every respect 

 to the recent shells of species which bear the same names 

 and still survive ; and it is impossible for the most critical 

 species-maker to distinguish one from the other. Even 

 their varieties and monstrosities or abnormal forms are 

 still repeated. The opinion of the late Professor D'Or- 

 bigny, which has been adopted by Agassiz, that all tertiary 

 species became extinct, and that they are only repre- 

 sented at present by analogous forms, evidently resulted 

 from a preconceived theory, against which a concordance 

 of fossil with recent species would have militated. 



Without, however, entering into an abstruse (and 

 perhaps useless) inquiry into the origin of species, or 

 how far they have been modified during any period of 

 time, however vast, by external circumstances or condi- 

 tions, it is undeniable that certain definite forms, called 

 " species," exist, and that they constitute more or less 

 extensive groups of individuals, which resemble each 

 other as well as their parents and offspring, to the same 

 extent that we observe in the case of our own kind. 

 These groups, to deserve the name of species, must 

 be distinct from others ; because if any of them are so 

 intimately blended together by intermediate links as to 

 make the line of separation too critical, the test fails, and 

 a subordinate group, or what is called a " variety," is 

 the result. For this reason it is indispensably necessary 

 to compare as great a number of individuals as possible, 

 and especially a series of different ages and sizes com- 

 mencing ah ovo, as well as specimens collected from 

 various localities. The study of abnormal or monstrous 

 forms is also important in order to ascertain the range 

 of variation in growth. By such investigations a crying 

 abuse of the present school of natural history (an ex- 

 cessive multiplication of species) would be avoided, 



