C— GEOLOGY 63 



of certain accessible structures ; there he can find facts, whereas his 

 excursions into phylogeny must always have some speculative element. 



This limitation is by no means so serious as it may appear. Any 

 organism consists of a mass of interrelated characters, each of which 

 should rightly contribute to the harmonious working of the whole. It 

 is obvious that many of the characters of an individual suffer change 

 during its lifetime, and that these changes are not attained at a uniform 

 rate. Indeed, individual life can be likened to a chord which is per- 

 sistently modulated by alteration in value of its component notes, until 

 the time comes when one or more of the notes is so altered that it produces 

 discord, a sure foreboding of disease and death. As a consequence, a 

 careful watch on the changes that affect a few characters will suffice to 

 show both the nature of such changes and their influence on the well- 

 being of the organism concerned. 



Palaeontologists thus study the history of organic structures rather than 

 that of organisms, thereby indirectly watching the fate of the owners 

 of those structures. In large measure the application of generic and 

 specific names (an arbitrary habit even in Neontology) is tentative. It gives 

 convenient, but often false, means of expressing morphological qualities. 

 Such familiar ' genera ' as Gryphcea and Exogyra can be shown to represent 

 stages in the morphogeny of oyster-shells belonging to manifestly different 

 lineages, so that they are not genera in any strict sense. They correspond 

 to such epithets as ' crony ' or ' gaffer ' as applied to stages in human 

 development. 



There is a wide range of variation in the durability of fossil types in 

 geological time. This variability affects all grades in classification except 

 perhaps the highest, and may be assumed, granted a sufficiently long 

 perspective, to affect all. Some classes, such as the Spire-bearing 

 Brachiopoda, lasted no longer than two eras, while others, such as the 

 Atreme Brachiopoda, have endured throughout the known record. The 

 families into which such classes are divided often show proportionate 

 durability ; the spire-bearing Atrypidae, for instance, being limited to 

 about two periods, while the atrematous Lingulidae have persisted from 

 Ordovician times to the present day. Similarly, the genera and species 

 of such families follow, in general, the fashion of the groups to which they 

 belong. If we consider a stratigraphical hemera as analogous with a year, 

 and a genus as an individual unit, it would be fair to recognise some 

 genera as annuals, some as biennials, and others as perennials of varied 

 longevity. It is worth noting that a precisely comparable variability of 

 expectation of life applies in the case of individuals ; so that, accidents 

 apart, an oak tree will live longer than a sycamore and a man than a mouse. 



Within the framework of a class there is actually much variability 

 of time-range. In the contrasted cases of the Brachiopoda cited above, 

 the family Spiriferidaj persisted through four geological periods, whereas 

 several families of atrematous Brachiopoda seem to have been limited to 

 the Cambrian period. Again, among the Echinoidea, the small regular 

 sea-urchin Hemipedina appeared at the outset of the Jurassic period, 



