218 SCIENCE PROGRESS 



return to tubercles and finally to a smooth stage in the animal's 

 second childhood. 



It is not my purpose on the present occasion to discuss 

 the bearing of these facts — for they are observed facts in spite 

 of the necessarily somewhat mystical style of their presenta- 

 tion — on current biological theory. Nevertheless it will be 

 obvious to all that they reveal the very great insufficiency of 

 many favoured hypotheses. 



The fact that the evolutionary change of any structure is 

 not haphazard but proceeds along a definite track gives us a 

 real foundation for the finer division of geological time on 

 palseontological evidence. We have only to take some phyletic 

 series which is really known, and divide the rocks in which the 

 remains of its individuals are found into zones on the stage 

 to which the evolution of some particular structure has pro- 

 ceeded. Simple as this process is in theory, it is impossible 

 of direct practical application, because of the difficulty of 

 distinguishing between the members of allied stocks pursuing 

 parallel courses of evolutionary change, a difficulty which 

 probably increases as the number of independent variables 

 which can be observed becomes smaller, being least, though 

 even then very serious, in vertebrates and increasing in echino- 

 derms, brachiopods, and cephalopods down to such things as 

 graptolites. 



In practice, in most cases it is necessary to use all members 

 of allied stocks which will differ to a greater or less degree 

 in their rate of change. There is evidence that an animal 

 which in any one structure has progressed more rapidly than 

 its relatives will have fallen behind them in other directions ; 

 a horse with unusually progressive teeth, for example, may 

 have retarded feet. 



In this way, by taking many species of allied stocks found 

 in the same bed and averaging up, first the stage of each and 

 then that of all, we may be able to found definite time divisions, 

 the size of which will depend on the closeness with which we 

 restrict the animals we use to a single phyletic stock. 



The more satisfactory attempts to " zone " a series of 

 rocks do really depend on the conscious or unconscious use of 

 this method, which is destined to play an increasing part in 

 stratigraphical geology as the number of students of modern 

 palaeontology increases and as the determination (with greater 



