430 J. BARRELL INFLUENCE OF CLIMATES ON A^ERTEBRATES 



grow great in body as compared to the invertebrates and teachable from 

 the gradual accumulation of experience. Seeing, then, how an organic 

 plan may have ultimately profound results not yet in evidence in the 

 actions and reactions which determined its beginning, what were the con- 

 sequences imposed by the Silurian-Devonian ganoid fishes on subsequent 

 evolution? In the adaptations recurrently necessary at that time for 

 crawling and breathing, in order that river fishes could maintain their 

 existence, what other methods, built up with slightly different habits, 

 would have been equally efficient, but which, so far as we can tell, might 

 have resulted in a lower, or possibly in a higher, future for vertebrates? 

 This is not an idle question, evoked only as a speculative fancy, since a 

 discussion of it gives a better insight into the causes and results of the 

 plan which was pursued. 



RELATIONS OF HABIT IN PRIMITIVE FISHES TO THE DEVELOPMENT OF 



PAIRED FINS 



In regard to the mode of locomotion, crawling is effectively done by a 

 snake-like or eel-like wriggling motion. This is a body form to which 

 several of the surviving archaic fishes have in fact approximated. But 

 the degeneracy of limbs which this mode of motion involves clearly cuts 

 off such animals as tend to pursue it from further participation in pro- 

 gressive evolution. The ancestors of amphibians were therefore fishes 

 with rather short, fishlike bodies and powerful paired fins. 



The early ganoids and dipnoans were provided with two pairs of lateral 

 fins, pectoral and pelvic. From these, although the stages have not been 

 found in the record of fossils, the four limbs were evolved, and thus the 

 number of limbs rests ultimately on the mode of swimming which called 

 for the existence of these four fins supplemental to caudal propulsion. 

 Modifications of habit in the ancestors of these fishes could conceivably, 

 however, have given to them and to their descendants, the amphibians, 

 either two, four, or six lateral appendages. What habits related to what 

 environments favored the establishment of the number of limbs as four? 



The bottom-living animals, such as illustrated by the eurypterids and 

 ostracoderms, find a powerful forward pair of limbs, acting as oars on the 

 bottom mud, sufficient for their needc. The origin of land-dwelling ver- 

 tebrates from such forms would have been a heavy ultimate handicap on 

 vertebrate evolution. The swimming types of fishes, on the contrary, 

 have customarily retained the four lateral fins. In Jie archaic types, 

 with an external primitive limb or archipterygium, the two pairs would 

 seem to have been effective for the multiple function of guiding in swim- 

 ming, and also rowing, or crawling through submerged vegetation. The 



