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THE AMERICAN NATURALIST [Vol. L 



small, smooth or unornamented species. The septae of 

 ammonites begin simple and later evolve their extraordi- 

 nary foldings. The horns of titanotheres "have exces- 

 sively rudimentary beginnings phylogenetically, which 

 can hardly be detected on the surface of the skull" (Os- 

 born, 1912, p. 253). Also, these "rudiments arise inde- 

 pendently on the same part of the skull in different phyla 

 at different periods of geological time." 



(b) The History of Progressing Phylogenetic Develop- 

 ment.— While paleontologists have no knowledge of 

 germinal conditions they can study the course of evolu- 

 tion of a particular character through a lineage compris- 

 ing thousands of generations. Paleontologists are 

 agreed that characters tend to become more and more 

 complex. So Beecher (1898, p. 354) writes: 



The smooth, rounded embryo or larval form [terms used in the phylo- 

 genetic sense] progressively acquires more and more pronounced and 

 highly differential characters through youth and maturity. In (paleon- 

 tological) old age, it blossoms out with a galaxy of spines, and with 

 further decadence produces extravagant vagaries of spines. So in the 

 titanotheres the horn rudiments evolve continuously, and they gradually 

 change in form, . . . they finally become the dominant characters of the 

 skulls, showimr marked variations of form in the two sexes. 



The saber-toothed and other tigers gained canines that 

 they could not use. The mollusc, Eippurites, gains a 

 shell a foot thick. In the Labyrinthodonts the infolding 

 of the teeth has been carried to an extraordinary degree, 

 etc. These are illustrations merely of what is said to be 

 a general rule. F. B. Loomis 1 writes of it under the 

 head "Momentum in Evolution." 



(c) The Irreversibility of the Process of Evolution 

 has been often remarked upon. It leads to exaggerated 

 developments in one direction; lateral and backward 

 variations are relatively uncommon. 



Thus, the horse has shown divergent lines of evolution 

 but none returning to the four-toed ancestral type. The 

 ammonites went from simpler forms of septum to more 

 and more complex without reversal, except at the very 

 end of their phylum. D. Rosa says : 



i Amer. Nat., 39 (1905). 



