242 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 34 



which may spend their entire lives swimming around in the water 

 without of necessity having to resort to the bottom. 



Shall we now have to consider all these old Paleozoic forms as 

 having been only secondarily benthonic? Do we really have to as- 

 sume that they were derived from earlier free-swimming ancestors? 

 To me the opposite thought seems much more natural, namely, that 

 the oldest fishlike forms and fishes were primarily bottom-living. 



We know very little about the ancestral forms of the vertebrates. 

 But we may take for granted that they evolved in water. It is then 

 also most natural to assume that the earliest, most primitive forms, 

 lived on the bottom and had not yet specialized sufficiently to be able 

 to swim. If the oldest vertebrates were bottom-living or perhaps 

 even burrowing forms, they must have learned to swim just as they 

 later had to learn to crawl, walk, run, and finally fly. 



In my opinion the oldest known vertebrates, fishlike forms, and 

 fishes, had not yet learned to swim, and we can in them observe the 

 gradual transition from bottom-living to free-swimming forms. The 

 clumsy forms with a large cephalothorax and a short thin tail, with- 

 out organs of steering and balancing, could only move rather 

 helplessly through the water and could scarcely lift themselves from 

 the bottom (figs. 3, 5, 6). The further evolution went in the di- 

 rection of the development of organs of equilibrium and of gliding 

 surfaces in the form of more or less strongly developed spines or 

 projections (fig. 3; fig. 7, A-E; fig. 8, A-D). The next step was the 

 formation of effective steering organs in the form of movable paired 

 fins (fig. 3, fig. 8, E). When all these technical difficulties had been 

 overcome, the further modifications continued only in the direction of 

 a more perfect adaptation of the entire body and of each separate 

 organ to the function of swimming. 



First of all, a gradual reduction of the heavy and thick external 

 armature or scales took place, thereafter a modification in the shape 

 of the body, and finally, a gradual perfection of the most important 

 organ of propulsion, the caudal fin. 



Parallel with these changes the inner skeleton also grew stronger 

 and came to form a strong support for the greatly developed swim- 

 ming musculation (fig. 12). We have seen the first step in this 

 evolutionary series in the Ostracoderms and Placoderms. The well 

 known evolutionary series of the Teleostomes show the subsequent 

 steps very clearly (figs. 1, 11, and 12). 



The oldest Devonian Crossopterygian (fig. 11, A; fig. 12, A) 

 still have a comparatively plump body, and their large heads are 

 covered by thick bony plates, their bodies by heavy scales. Their 

 paired fins are brush-shaped and cannot be closely applied to the 

 body, which fact serves to prevent very rapid swimming. Their 

 heterocercal tail has a strong, scaly axis. They are fair but not yet 



