THE MAIN LINES OF ANiMAL EVOLUTION 



a comparison of the Tornaria and Bipinnaria larvae, has already been 

 discussed. 



As the phylum Chordata, including as it does the vertebrates and man, 

 will be discussed in the next chapter, it need not be taken up here beyond 

 the obvious statement that this phylum forms the climax not only of the 

 obscure (from the viewpoint of relationships) Deuterostomia, but also, 

 perhaps, of the entire Animal Kingdom. Figure 50 summarizes one view- 

 point on animal phylogeny. 



SOME EVOLUTIONARY GENERALIZATIONS 



Now that the greater part of the course of evolution has been sketched, 

 and before taking up that part of the story which is most closely related 

 to those who study it, it may be well to give some consideration to broad 

 tendencies and principles of evolution. At the outset, evolution is not al- 

 ways "upward" or "progressive." Many examples are known of the evolu- 

 tion of simpler or more degenerate types from originally complex types. 

 Thus the fungi may have evolved from algae by the loss of chlorophyll. 

 Grasses have evolved from lily-like ancestors by simplification of parts, 

 especially of flowers. Mistletoe, an angiosperm parasitic upon trees, has 

 undoubtedly evolved from free-living ancestors. Similarly in the Animal 

 Kingdom, many examples of retrogressive or degenerative evolution are 

 known. The development of sexual reproduction and its great evolution- 

 ary importance was emphasized in Chapter 8. The Rotifera were 

 undoubtedly evolved from bisexual ancestors, and some species are still 

 bisexual. Nonetheless there are species in which the males are unknown. 

 These still reproduce sexually, for the ova develop parthenogenetically, 

 but the major advantages of sexual reproduction are lost. The develop- 

 ment of the parasitic habit almost always involves degenerative evolution. 

 In the tapeworms, this is extreme. Although derived from free-living flat- 

 worms, with well-developed digestive, nervous, reproductive, muscular, 

 and other systems, the tapeworm is reduced substantially to an absorptive 

 sac containing gonads. But, while such degenerative evolution is charac- 

 teristic of parasites, it is by no means confined to them. As already pointed 

 out, the class Archiannelida was most probably derived from polychaete 

 ancestors by a process of simplification and loss of parts Thus it is clear 

 that evolution can be retrogressive as well as progressive 



Origin of New Groups from Primitive Ancestors. A second very 

 important generalization relates to the form of the tree of life. It is more 

 properly a shrub than a tree, for new groups do not arise from the most 

 advanced and specialized members of their parent groups, but from the 

 primitive, unspecialized ones. Thus the primitive flagellates have given 

 rise to many additional plant and animal groups, but the more specialized 

 protozoan and algal groups are generally terminal. Again, if indeed the 

 Hemichordata and Chordata were derived from echinoderms, it seems 

 certain that the more advanced phyla must have arisen from the ancestral 

 Dipleurula in a very primitive stage before radial symmetry developed. 

 One more qualification relative to the shape of the phylogenetic tree is 



151 



