and Embryological Development. 371 



semblance of truth ; they lead us to branchlets having but a 

 slight trace of probability, to branches where the imagination 

 plays an important part, to main limbs where it is finally 

 allowed full play, in order to solve with the trunk, to the 

 satisfaction of the writer at least, the riddle of the origin of 

 the group. It seems hardly credible that a school which 

 boasts for its very creed a belief in nothing which is not 

 warranted by common sense should descend to such trifling. 



The time for genealogical trees is past ; its futility can, 

 perhaps, best be shown by a simple calculation which will 

 point out at a glance what these scientific arboriculturists are 

 attempting. Let us take, for instance, the ten most charac- 

 teristic features of Echini. The number of possible com- 

 binations which can be produced from them is so great that 

 it would take no less than twenty years, at the rate of one 

 new combination a minute for ten hours a day, to pass them 

 in review. Remembering now that each one of these points of 

 structure is itself undergoing constant modifications, we may 

 get some idea of the nature of the problem we are attempting 

 to solve when seeking to trace the genealogy as understood by 

 the makers of genealogical trees. On the other hand, in 

 spite of the millions of possible combinations which these ten 

 characters may assume when affecting not simply a single 

 combination, but all the combinations which might arise from 

 their extending over several hundred species, we yet find that 

 the combinations which actually exist (those which leave 

 their traces as fossils) fall immensely short of the possible 

 number. We have, as I have stated, not more than twenty- 

 three hundred species actually representing for the Echini 

 the results of these endless combinations. Is it astonishing, 

 therefore, that we should fail to discover the sequence of the 

 genera, even if the genera, as is so often the case, represent, 

 as it were, fixed embryonic stages of some Sea-urchin of 

 the present day ? In fact, does not the very history of the 

 fossils themselves show that we cannot expect this ? Each 

 fossil species, during its development, must have passed 

 through stages analogous to those gone through by the Echini 

 of the present day. Each one of these stages at every 

 moment represents one of the possible combinations ; and 

 those which are actually preserved correspond only to the 

 particular period and the special combination which any Sea- 

 urchin has reached. These stages are the true missing 

 links, which we can no more expect to find preserved than 

 we can expect to find a record of the actual embryonic de- 

 velopment of the species of the present day without direct 

 observation at the time. The actual number of species in 



