8 



pattern is discovered. That patterns quite distinct from those known to zool- 

 ogists have existed in past ages has been well proven by paleontologists. 

 How can the structures of a species of such a kind be inferred from a frag- 

 ment ? Another law equally true with that of persistence has been developed 

 from the facts, but it is much more difficult of application. This is the one 

 already defined by the writer^ under the name of the law of " successional 

 relation.'' It is absolutely certain that the types of nature, whether pri- 

 mary or subordinate, form series of steps passing from one condition of rela- 

 tions to another. The natural deduction is, that if a portion of an animal 

 exhiljits a form intermediate between two known forms or types, the remainder 

 of the animal's structure possesses the same kind of intermediacy. This 

 law is tacitly admitted, and employed by paleontologists ; but there is a diffi- 

 culty of application in consequence of the existence of other laws now to be 

 considered. 



The first difficulty arises from our possible ignorance of one terminus of 

 the series or hue in which our fossil represents a stage. This objection is 

 more theoretical than real, because the living classes and orders are the struc- 

 tural extremes of the lines of succession ; nevertheless, among divisions of 

 lesser range, many have reached their culmination, and disappeared in times 

 past. These points of culmination must be known in order to ascertain the 

 direction of the succession. Every discovery, however, is not that of an 

 advanced position on such lines ; intermediate positions being necessarily more 

 numerous than termini. Hence, this difficulty is of only occasional recurrence. 



The preceding considerations all express different phases of the law of 

 uniformity. I now refer to the law of variation, which is in apparent conflict 

 with it. It is the law which expresses evolution as opposed to persistence of 

 types. It especially limits the application of the last law, that of uniformity 

 in succession ; i. c, that when one portion of structure occupies a position 

 intermediate between two already known types, the remaining parts of the 

 same animal or system of organs will occupy the same relation of structure 

 to the corresponding parts of the known. This is not uniformly true. The 

 law of variation intervenes, which states that it may occur that, while one 

 part of an organization occupies a relation of intermediacy, the other parts do 

 not exhiliit exactly the same relation. It is by the unequal mingling of 

 structural points that new lines of succession are marked out. Thus it is 



' Pcuii Monthly Magazine, 1872, p. 229. 



