THE PROBLEMS OF MORPHOLOGY. 11 



the sub-branches representing allied genera. Similarly with 

 the ancestral types of classes, still deeper down in the tree or 

 further back in time. So that phylogeny becomes more and 

 more speculative as its questions become more and more 

 radical. And the difficulty is made greater by the deficiency 

 of palaxmtological evidence. 



One obvious corollary is that an ancestral type from which 

 sundry allied types now existing diverged, was, speaking 

 generally, simpler than these; since the divergent types be- 

 came different by the superposing of modifications, adding 

 to their complexities. There is a further reason for inferring 

 that the least specialized member of any group is more like 

 Hit 1 remote ancestor than any of the others; for every adap- 

 tation stands in the way of subsequent re-adaptations: it 

 presents a greater amount of structure to be undone. To get 

 some idea of the ancestral type where no extant member of 

 the group is manifestly simpler than the rest, the method 

 must be to take all its extant members and, after letting 

 their differences mutually cancel, observe what remains com- 

 mon to them all. 



But there are difficulties standing in the way of phylogeny, 

 and consequently of morphology, much greater than these. 

 Returning to our symbolic tree, it is clear that it would be 

 far from easy to say of any one twig which extinct sub- 

 branch, branch, and main branch it belonged to, even sup- 

 posing that the growths of all parts had been uniformly out- 

 wards. Immensely more perplexing, then, must be the 

 affiliation if various of the branches, sub-branches, &c, have 

 sent out backward-growing shoots which have come to the 

 surface only after prolonged retrograde courses, and if other 

 branches have sent shoots into regions occupied by alien 

 branches — shoots bearing twigs which come to the surface 

 along with those to which they are but remotely allied. The 

 problems of origin and of structure which organisms present, 

 are met by both of the difficulties thus symbolized. 



One of them arises from the prevalence of retrograde 



