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 supposing that the growths of all parts had been uniformly outwards. 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 metamorphoses. Throughout the animal world these are variously displayed by parasites, multitudinous in their kinds; for most of them belong to types much higher in organization. Changed habits and consequent changed structures have so transferred them that only by study of their embryonic stages can their kinships be made out. And these retrograde metamorphoses, conspicuous among parasites, have, in the course of evolution, affected some members of all groups; for in all groups the struggle for existence has compelled some to adopt careers less trying but less profitable.