22 BIOLOGICAL LECTURES. 



through the repeated and long-continued action of the waves, 

 are beaten into fragments mingled in the most inextricable con- 

 fusion, until eventually covered by a deposit of sand or clay. 

 Through all the long period of this process, decay has wrought 

 destruction in the more perishable material, until that which 

 finally passes into the fossil state represents a mere fragment 

 only of the original material. The significance of such evidence 

 becomes apparent when it is desirable to employ such remains 

 as a test of climate, since the conclusions drawn must obviously 

 depend upon whether the material is in situ or not, and if not, 

 its original source. 



Fossil plants are very rarely, if at all, found in anything like 

 a complete condition. This naturally results from the destruc- 

 tive action of the water which serves to transport them, as well 

 as from the unequal effects of decay and the operation of pres- 

 sure and heat to which they are so often subjected. From this 

 it follows that the mere association of parts, when organic union 

 is wanting, is altogether untrustworthy as evidence of common 

 origin or specific identity ; and it most commonly happens that 

 the restoration of a species can be accomplished, if at all, only 

 after an exhaustive study of numerous fragments from different 

 sources. Few such restorations are satisfactory ; most of them 

 are purely hypothetical with respect to many of their most 

 important features, and they thus serve no final purpose; they 

 are merely the expression of what we commonly term "work- 

 ing hypotheses." Large numbers of plants are known by their 

 leaves only. This is particularly true of the Tertiary, as also 

 of the Cretaceous flora, where, however, the fruit and sometimes 

 the silicified or calcified stems also aid in identification. In 

 the Devonian and Silurian they may be known by leaves, but 

 more commonly by fragments of branching stems, by fruits, 

 or by exceptionally well preserved stem structure. In recent 

 formations like the Pleistocene, the stem structure may be so 

 perfectly preserved as to admit of direct and exact comparison 

 with existing species, and thus the genus, and often the species 

 itself, may be determined beyond all reasonable doubt. It will 

 be observed, however, that in the preservation of those parts, 

 upon which the determination of relationship so largely depends, 



