2 The Value of Paleontology. 
This law of persistency presupposes a knowledge of the pattern 
as essential to its deductive application. Hence a difficulty at 
once suggests itself as arising when a portion of an animal belong- 
ing to a new pattern is discovered. That patterns quite distinct 
from those known to zodlogists have existed in past ages, has been 
well proven by paleontologists. How can the structures of a 
species of such akind be inferred from a fragment? 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 in the pages of this journal,” under the name 
of the law of ‘successional relation.’’ It is absolutely certain 
that the types of nature, whether primary or subordinate, form 
series of steps passing from one condition of relations to another. 
The application evidently is, that if a portion of an animal ex- 
hibits 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 pa- 
leontologists, but there is a difficulty of application in conse- 
quence 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 line in which our fossil represents a 
stage. This objection is more theoretical than real, because the 
living classes and orders are the structural 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; 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 evolu- 
tion as opposed to persistence of types. It especially limits the 
application of the last law, that of uniformity in succession, 7%. ¢., 
that when one portion of structure occupies a position interme- 
diate between two already known types, the remaining parts of 
the same animal or system of organs will occupy the same relation 
?Penn Monthly, 1872, p, 229, 
