FINAL REMARKS 497 



with the result that, in the minds of many, embryology has failed to 

 afford the desired clue. 



At the same time, the geological record was looked upon as too 

 imperfect to afford any real help ; it was said, and is said, that the 

 Cambrian and pre-Cambrian periods were so immense, and the animals 

 discovered in the lower Silurian so highly organized, as to compel 

 us to ascribe the origination of all the present-day groups to this 

 immense early period, the animals of which have left no trace of 

 their existence as fossils. 



In consequence of, or at all events following upon, the supposed 

 failure of embryology and of geology to solve the problem of the 

 sequence of evolution of animal life, a new theory has arisen, which 

 goes very near to the denial of evolution altogether. This is the 

 theory of parallel development. It discards the old picture of a genealo- 

 gical tree with main branches arising at different heights, these again 

 branching and branching into smaller and smaller twigs, and substitutes 

 instead the picture of the ribs of a fan, every rib running independently 

 of every other, each group represented by a rib reaching its highest 

 development on the circumference of the fan and coming nearer 

 and nearer to a common point at the handle of the fan. This point 

 of convergence, where all the groups ultimately meet, is so far back 

 as to reach'to the lowest living organisms. 



This, in my opinion, unscientific and inconceivable suggestion has 

 arisen largely in consequence of a conception which has become 

 firmly fixed in the minds of very many writers on this subject — the 

 conception that in the evolution of every group, the higher members 

 of the group are the most specialized in the peculiarities of that group, 

 and it is impossible to obtain a new group with different peculiarities 

 from such specialized members. If, then, a higher group is to arise 

 from a lower, it must arise from the generalized members of that 

 lower group, in other words, from the lowest members or those 

 nearly akin to the next lower group. 



Similarly, the highest members of this latter group are too 

 specialized, and again we must go to the more generalized members 

 of the group. In this way each separate specialized group is put-on 

 one side, and so the conception of parallel development comes into 

 being. 



The evidence given in this book dealing with the origin of 

 vertebrates strikes at the foundations of this belief, for it presents an 



2 K 



