362 AMPHIBIA xiii. 4 



4. Tendencies in the evolution of fossil Amphibia 



The changes in the form of amphibia can be followed from the 

 beginning of the Carboniferous to the end of the Triassic period, and, 

 indeed, in the form of their reptilian descendants far beyond. There 

 are signs, however, of very many distinct lines (as we should expect), 

 and it is not possible to trace details of the history or fate of par- 

 tieular populations. Two distinct tendencies appear over this period: 

 (i) to become fully terrestrial, (2) to return to the water. The terres- 

 trial forms became very gradually shorter in body and stronger in leg. 

 The skull remained fairly high and domed and the otic notch became 

 deeper, as a more effective tympanum developed. In the vertebrae 

 the hypocentrum became reduced as muscles developed attached to 

 the pleurocentrum. 



Return to the water led to animals of two distinct types, (a) snake- 

 like or (b) flattened, but in both there was a reduction of limbs and 

 a secondary lengthening of the body, with return to the sinuous 

 movements of fish-like locomotion. In the bottom-living forms, such 

 as some Stereospondyli and *Diplocaahis among Lepospondyli, the 

 skull became flattened, with the eyes looking upwards, the otic notch 

 being shallow. The snake-like *Dolichosoma retained the more normal 

 skull shape but became immensely elongated and lost the limbs 

 altogether. 



These observed tendencies can be understood to result from the 

 situation that developed as the vertebrates first colonized the land. 

 The earlier amphibia, such as *Eogyrinus, were partly aquatic, by 

 force, one might say, of inexperience. Throughout the Carboniferous 

 various lines of them became more fully equipped for terrestrial life, 

 moving faster, seeing and hearing better, and so on. The competition 

 and predatory attacks of these more successful lines then drove others 

 back into the water and so the process continued, until later the earlier 

 reptilian lines, themselves driven back to the water by their own 

 descendants, removed most of the amphibians from the waters as 

 well as from the land, leaving only some few remaining populations, 

 from which the modern orders have evolved. 



It certainly does not seem necessary to postulate any special 

 directive force to explain all this. We could wish, of course, for 

 much more information, but it seems reasonable to imagine that 

 these changes were produced by the action of the animals with each 

 other and with the environment, supposing that the animals con- 

 tinually strive to feed, grow, and reproduce others rather, but not 

 quite, like themselves. 



