THE ORGANIC WORLD 911 



such cases we are certain to get a complete record of the individual 

 development of the animal from the youngest stages in which such 

 structures are formed to the adult or even old age stage. Thus, in 

 the case of the molluscan shell, for example, we have a complete 

 record of the individual dcvelojMiient or ontogeny, ail the changes 

 l)eing indicated in the successive -whorls or areas of the shell, from 

 the initial shell-plate to the adult. Moreover, such shells will 

 I)reserve the detailed features assumed necessarily by the mantle 

 of the animal in conformity with development and increase of in- 

 ternal organs, features which are evanescent in the soft parts and 

 can in many cases not even be observed. 



What has just been stated is true mainly of animals whose 

 hard parts are external structures, retained throughout life and 

 increased by addition in one region only. Such additions are recoi^- 

 nizable by the formation of growth lines, and they are typically 

 shown in the shells of the Mollusca, as already noted. Rrachiopoda 

 also show such changes in external form but there are, in addition, 

 changes in interior structures, for the study of the development 

 of which a complete series of individuals, representing all the 

 stages, is needed. Corals likewise show their growth lines, and 

 sections made across the older part in most cases show the prog- 

 ress of development of the individual. Other invertebrates are less 

 satisfactory. While the serial development of parts can be made 

 out in the plates of an adult echinoderm, yet the fact that each 

 plate changes with the progress of development interposes certain 

 difficulties in the study of the life history, and for its complete 

 determination individuals of various age stages are needed. This 

 is also true of the vertebrated animals where the internal skeleton or 

 external armor changes with the growth and development of the 

 individual, and where, therefore, skeletons of the young as well 

 as the adult are needed to determine the entire life history. 



Plants and Animals as Indicators of the Age of the Period 

 IN Which They Occur. 



Since we have realized, by a prolonged study of modern as 

 well as ancient organisms, that animals and plants have gradually 

 increased in complexity of structure and diversity of form, from 

 the earliest times to the present, it has become possible to use the 

 remains of organisms embedded in the strata as indices of the 

 chronology of the earth's history, and by extensive collection of 

 facts from all geological levels, and over wide areas, to build up an 



