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TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



the survival of those larval fishes in which the union of the right and left halves of the 

 spinous dorsal was more and more delayed and in which the principal direction of growth 

 became transverse rather than vertical. 



An alternative hypothesis is that the ancestral echineid at first simply followed closely 

 in the wake of the shark and then moved up under its ventral surface, gradually learning to 



Echeneis naucrates 



Fig. 198. Echtneis. Top view. 



"steal a ride" by pressing its short spinous dorsal against the underside of the host. In this 

 connection Dr. William Beebe (1932) has called attention to the presence of numerous 

 suckers on the dorsal surface of the protruding lower lip of the larval Remora- remora as 

 probably serving for attachment to the host before the dorsal sucker was fully developed. 

 It is quite possible however that this is a purely caenogenetic or larval adaptation without 

 special phylogenetic significance. 



In any case, the evolution of such an indubitably well adapted organ as the sucking-disc 

 from a structure with quite different functions constitutes strong evidence for the potency 

 of Natural Selection in controlling changes in the direction of eyolution, by the selection 

 of indiscriminate hereditary variations with reference to their utility in a given situation. 



