CONSEQUENCES IMPOSED BY GANOID FISHES 431 



body form of the early crossopterygians, which, like other fishes, possessed 

 lateral flexibility, but vertical rigidity, obviated the need of more than 

 four such appendages for guidance or body support, as contrasted to the 

 numerous appendages needed to support the vertically flexible body of 

 arthropods. It was the habit of active river fishes, then, which deter- 

 mined the quadrupedal nature of land vertebrates. A closer study of the 

 relations of habits and environment of river fishes to the use of fins would 

 be instructive toward restoring this stage in the geological history of 

 vertebrates with more certainty than can now be done. 



Two pairs of lateral fins were sufficient to meet the demands of the 

 Silurian and Devonian environment. It is a suggestive question, how- 

 ever, if the development of habits leading to six initial limbs would not 

 have been an ultimate advantage. The insects rising from a many-legged 

 stock have permanently retained six as the minimum advantageous num- 

 ber with the exception of certain butterflies in which the forward pair 

 has become vestigial. With six limbs in the vertebrate body-plan, four 

 might have been retained for body support and purposes of locomotion, 

 and two could have been diverted readily and in many instances for other 

 uses, especially able, as arms and hands, to serve the higher needs of the 

 organism and stimulate the mental development. Such a stimulation 

 and wider competition would have tended apparently to introduce the 

 mental factor as of importance at an earlier era in evolution. 



There was a time in vertebrate evolution when perhaps six limbs were 

 still a possibility. The acanthodean sharks show a lateral line of spines 

 between the pectoral and pelvic fin spines. The Proselachii, as seen in 

 Cladoselache, show traces of a lateral fin fold. Specialization in the early 

 elasmobranchs, however, reduced these possibilities of multiple limbs to 

 the limited number of four. What habits in the ancestral chordate paral- 

 leled the development of four rather than of two or six primitive limbs? 

 To sum up the preceding discussion, the answer would seem to be a habit 

 of vigorous swimming, but close to the bottom, developed in connection 

 with caudal propulsion. This habit was developed in connection with the 

 possession of a notochordal axis between the alimentary and neural canals, 

 a body plan tending to vertical stiffness, but lateral flexibility. The 

 passage of the four lateral fin's into the fringed fins of the early crossop- 

 terygians. taken into connection with the knowledge of their fluviatile 

 habitats in semi-arid climates, was apparently due to life in shallow river 

 waters. In the time of flood these spread widely over the river plains. 

 In such shallow floodplain and swamp waters much tangled vegetation 

 grows, and the fins, tq aid in the progress of the animal by subaqueous 

 crawling through this ~ fetation, became elongated as fringed limbs. 



XXXI— Bull. Geol. Soc. Am., Vol. 27, 1915 



