PALEONTOLOGY 301 



never received accurate definition or interpretation. The 

 group may appear in a series of relatively simple forms, showing 

 affinities with some type from which it may have diverged. 

 These early genera will be succeeded in the rocks by others, 

 arranged progressively so as to form a series apparently mov- 

 ing in a certain direction. Each genus successively following 

 in time, will perhaps show a greater and greater emphasis on 

 some one group of characters, a greater and greater specializa- 

 tion in some one direction. Arranging the genera in series, it 

 looks as if there were a definite line of variation shown in their 

 gradual succession. These phenomena have been shown in 

 various groups of reptiles and fishes, and especially well in the 

 evolution of the extinct order of ammonites. These animals, 

 allied to the living nautilus, lived in coiled chambered shells 

 which gradually assumed great complication of form and orna- 

 mentation. The extreme of this course of evolution was fol- 

 lowed by corresponding progressive degeneration. In some 

 cases, this condition continues to the present time. More 

 frequently, the specialization along the original lines continues 

 to a certain point, to be followed by the progressive degenera- 

 tion and perhaps the ultimate loss of the very same structures 

 in which the high degree of specialization has prevailed. 



To phenomena of this kind, the term determinate varia- 

 tion or orthogenesis has been applied. This phrase seems to 

 involve the theory that the evolution has gone forward toward 

 some predetermined end, or that in some way only variations 

 leading toward this end have existed or at least have been 

 able to maintain themselves. It is possible, however, that the 

 cause may be found in the influence of some phase of environ- 

 ment, which directs the course of natural selection continuously 

 along a certain line. A reversal of selection would be naturally 

 followed by a degeneration of the structures developed to a 

 point beyond the need of the animal. 



It is plain that much is to be learned, especially in regard to 

 the relationships existing among living animals, by a study of 

 those of the past. A comparison of certain of the ancient 

 reptiles with the long-tailed Archccopteryx (Fig. 178) and other 

 toothed birds shows that the birds and reptiles were once 

 scarcely distinguishable, although now so very different. Birds 

 have feathers, reptiles do not; but there is scarcely any other 

 permanent difference. Fossils show a similar close relation 



