REPORT OX THE ECHINOIDEA. 19 



indications of the outlines of the relationship ; and the very fact that this systematic 

 relationship can still be traced so satisfactorily, not only at the present day, but even 

 in past geological periods, shows us plainly that this range of variations of our twenty 

 variables is far less great than is possible, and is kept within comparatively narrow 

 bounds, otherwise the possible combinations would far exceed our ability to trace them. 



In fact, in many classes of the animal kingdom the task of tracing their affinities 

 and reducing them to the factors from which they originated by following the combina- 

 tions, appears on the face of it a puzzle far exceeding our ability to cope with, and we 

 might as well recognise the very narrow limits within which this problem has any 

 solution. We are brought at once face to face with the number of definite things which 

 we are able to carry in our mind at one time ; this number is quite limited compared 

 to the possible combinations which even the smallest number of variable factors repre- 

 sented by the changes the component structural features of any small group of animals 

 may assume. Supposing that for twenty years we became acquainted with one species 

 a minute for ten hours a day, we should not know as many possible combinations as can 

 be formed out of ten such variables as I have mentioned, which affect radically the 

 facies of any one of our 225 genera of Echinoidea ; and taking it for granted that the 2300 

 known species of fossil and recent Echinids are the only combinations which become 

 sufficiently permanent to have transmitted their principal characteristics for a certain space 

 of time sufficiently long to be entitled to recognition as distinct species. We must also 

 remember that the affinities they represent are the result of a far greater number of 

 possible comliinations than those to which I have referred, and that even a limited num- 

 ber of species like this baffies all our attempts at indicating these affinities, except in 

 the most general way ; or, putting it in a different manner, we are attempting an integ- 

 ration within very distant limits, and are, of course, trying to solve a most difficult 

 problem, which is not a whit nearer its solution by being presented in the customary 

 diagrammatic form of a genealogical tree, no matter how satisfactory this mode of 

 presenting the affinities of the group may appear to its author. But I wish at the same 

 time to be distinctly understood as not calling in question in the least the theory of the 

 direct succession of the Echinids of the present epoch from those of the Chalk, in spite of 

 the hopeless nature of the attempt to represent this succession, either diagrammatically 

 or descriptively. 



Relations of the Jurassic Echinoidea to the Echinid Fauna of the present day. 



Starting from the Jurassic Pygaster, which stUl has the closest possible relations 

 to the Desmosticha, in which the anal system has passed into the odd posterior interam- 

 bulacrum, we can readily trace the systematic connection to such forms as IIoIecti/2)iis, 

 Discoidea, Corwclyptis, in which the true Clypeastroid features are more and more 



