CONSEQUENCES IMPOSED BY GANOID FISHES 429 



out, then the ability to breathe air would be an advantage, enabling the 

 possessors of that power, although gasping for breath and half asphyx- 

 iated, to live where other fishes died in masses. A long period of strenu- 

 ous evolution is implied, the pressure of natural selection eliminating 

 continuall}' those not able to survive. Such internal changes of body 

 plan, involving an initial inefficiency of function, require an external 

 compulsion. The nature of that external force is found in the habitat 

 of the Devonian ganoids and the compulsion of seasonal dryness. 



Consequences imposed by Ganoid Fishes on subsequent Evolution 

 ultimate consequences of types of structure 



In the progress of evolution during the course of geologic ages many 

 paths have opened up. The hosts of living things have spread out and 

 occupied all to which they could become adapted. Each path has deter- 

 mined to a greater or less degree the subsequent journey of the race which 

 starts on its course, but, like a labyrinth, the end could not be seen from 

 the beginning. Many a wide and easy road has led to no further prog- 

 ress ; many a promising way has ended in extinction ; a few paths, narrow 

 in their beginnings, have led by devious and difficult ascents to higher 

 planes of life. 



For illustration, if a reflecting intellect could have looked on the world 

 at the opening of the Paleozoic; to it the exoskeleton of the arthropods 

 would have seemed a happy combination, suited both for protective 

 armor, weapons, and a muscular framework, marking apparently that 

 phylum as the one to contain the rulers of the earth through all future 

 time. During the Cambrian the animals possessing this organic plan 

 were, in fact, the dominant life of the seas. But time made evident a 

 fatal limitation ; the exoskeleton was a non-living secretion and therefore 

 could not grow. Frequent moulting was necessary with its exhausting 

 struggle out of the old shell, the rapid growth before the new should 

 harden, the helplessness of the soft-shelled stage. These conditions im- 

 posed the necessity of limiting the number of moults and keeping the 

 body small. An escape from the dangers inherent in moulting was found 

 in the development of metamorphosis; but this organic transformation 

 precluded a continuity of experience during growth. Without such con- 

 tinuity there could not be teachableness. The completed insect must be 

 an automaton, actuated from the beginning by the blind impulses of 

 instinct. 



In chordates, however, the decreasing emphasis on protective armor 

 and the development of a living endoskeleton permitted them in time to 



