60 SECTIONAL ADDRESSES 



such as angular unconformity, defaces the record, there is no reason to 

 assume that the story is consecutive. Just as a net has been described 

 as a set of holes held together with string, so a series of strata must 

 often represent a succession of non-sequences separated by films of 

 sediment. 



In addition to the accidents of destruction inherent in the nature of the 

 geological record, there are many gaps due to biological factors. Not 

 only do organisms devoid of hard parts perish, usually without trace, 

 but many of all kinds are destroyed in providing food for their successors. 

 The biological palimpsest immortalised on Ilkla Moor is almost universal. 



Apart from accidental occurrences that are too rare to provide more 

 than surprise, fossils consist of the ' hard parts ' only of the creatures they 

 represent. While in some cases these structures may consist of toughened 

 organic material (as, for example, wood or chitin), they are usually 

 built of mineral matter secreted or excreted by the organism. Such shells 

 and skeletons are valuable to their owners for protection or support ; but 

 at best they have a secondary significance in that they are the least ' alive ' 

 parts. Skeletons are closely associated, and intergrown, with living 

 tissues ; but shells have no closer connection with their builders than any 

 other kind of homespun garment. 



There is thus a serious limitation, in both quantity and quality, of the 

 amount of direct evidence available for the appraisement of the characters 

 of extinct organisms. Paleontologists share with anatomists the dis- 

 advantage of studying life after it has gone ; but they are further penalised 

 by having access only to those parts of the living mechanism that were 

 never more than half alive. When we superpose on this Ossa of imper- 

 fections the Pelion of the human factor (in the matter of collection, pre- 

 servation and interpretation of specimens), there does not seem much left. 



As regards the quantity of evidence available, its limitation is our 

 salvation. However short it may fall of the total amount possible, it is 

 enormous. In many respects we understand the principle by which it 

 has been selected, so that we are in a position to estimate its proportionate 

 value. Moreover, such material as is preserved for us has been kept in 

 its right chronological order. The fact of succession gives ample com- 

 pensation for shortcomings in other ways. 



The quality of fossil evidence is, in effect, far higher than might be 

 expected. Although fossils represent but portions of organisms, they are 

 not therein unrepresentative. In the laboratory of research in Scotland 

 Yard, a mere finger-print is known as a sure criterion for identification. 

 A finger-print suffices not only to show some inborn and peculiar character 

 of its maker, but often includes features that reveal something of his 

 habits and experiences. Most fossils, certainly those on which conclusions 

 of importance are based, are far more than finger-prints. In spite of a 

 need for caution owing to the vagaries of convergent development, a single 

 character is generally enough to serve as a basis of identification of an 

 organism. The plumage of birds, the wing-scales of Lepidoptera, or the 

 pollen-grains of plants are even better indices of the several species of a 

 group than many more intimate anatomical features. Indeed, experience 

 shows that ' vital ' structures are very uniform throughout families or even 



