PHYLOGENY 



to the water and assumed alga-like appearances, but their morphology is 

 still that of flowering, vascular plants. The evidence indicates that major 

 evolutionary steps, once taken, are never reversed. This is known as 

 Dollo's Law. It even might be expected a priori, for major evolutionary 

 steps are compounded of many smaller steps, each preserved by natural 

 selection. That such a sequence, occurring by chance once, should by 

 chance be exactly reversed would be a most extraordinary thing. If not 

 impossible, it is at least most improbable for whole organisms. Attempts 

 to apply Dollo's Law to individual characters have failed, for these are, 

 indeed, reversible by mutation. 



The Significance of Extinction. Some closing remarks should be made 

 upon the subject of extinction, for this has been the fate of most species 

 since the origin of life. Extinction may have completely different signifi- 

 cance in different instances. The dinosaurs were a highly specialized line 

 of Mesozoic reptiles which dominated the earth for a long time. But when 

 conditions became unsuited to their survival, they became extinct and left 

 no descendants. They were succeeded by other unrelated forms. The 

 cynodont reptiles also became extinct, but they were succeeded by their 

 own descendants, the adaptively superior mammals. Thus extinction may 

 mark the end of a line of descent or it may be the accompaniment of the 

 origin of new and superior types. It ought to be added that the major 

 adaptive types— phyla and classes— very rarely become extinct. 



These, then, are some of the conclusions which may be drawn from a 

 study of the course of evolution. The causative factors which have led to 

 these results will be taken up in parts III and IV. 



REFERENCES 



BoRRADAiLE, L. A., and F. A. Potts, 1958. "The Invertebrata," 3rd Ed., Macmillan 



Co., New York, N.Y. An authoritative and comprehensive text on the invertebrates. 

 Hadzi, J., 1953. "An Attempt to Reconstruct the System of Animal Classification," 



Systematic Zool., 2, 145-154. A radical revision, yet to be adequately assessed by 



other zoologists. 

 Hardy, A. C, 1953. "On the Origin of the Metazoa," Quart. J. Microscop. Sci., 94, 



441-443. A brief but stimulating paper urging the origin of the Metazoa from plants. 

 Hyman, Libbie H., 1940. "The Invertebrates," McGraw-Hill Book Co., New York, N.Y. 



Five volumes of this series have been published, and at least three more are planned. 



As far as it goes, it is the best, most comprehensive, and most profound treatment of 



the invertebrates in the English language. ( Cleveland, Kofoid. ) 

 IvANOV, A. v., 1955. "The Main Features of the Organization of the Pogonophora," 



Systematic Zool., 4, 170-178. New phyla are rarely described, but this is such a 



description. 

 Marcus, E., 1958. "On the Evolution of Animal Phyla," Quart. Rev. Biol, 33, 24-58. 



A scholarly presentation of a phylogcny similar to the present one in general, but 



differing in important respects. 

 Simpson, G. C, 1953. "The Major Features of Evolution," Columbia University Press, 



New York, N.Y. The viewpoint of a genetically minded paleontologist. This book 



includes much material applicable to the last part of this chapter. ( Cope, Dollo. ) 

 TiEGS, O. W., and S. M. Manton, 1958. "The Evolution of the Arthropoda," Biol. 



Rev., 33, 255-337. A penetrating review for mature students. 



154 



