Page:Popular Science Monthly Volume 73.djvu/567

Rh dorsal frill as anything but a hindrance to any quick movement. Cope suggested, in a spirit of fun, that these animals were the precursors of the modern fin-keeled yacht and that when they wished to navigate the Permian waters they swam upon their backs. Other authors have suggested equally abused uses in a similar spirit, but there are very few that can be considered with any degree of seriousness. The obvious suggestion is that the spines served as some form of protective mimicry, perhaps helping the animal to remain concealed among the reeds which bordered the lakes or streams, but this seems hardly necessary when we reflect that the animal was the dominant form of its time and needed no concealment unless it was to aid in lying in wait or in making an unseen approach until sufficiently near for the final rush upon its unsuspecting prey. This last is perhaps a fair suggestion, but it seems that the physiological burden of maintaining such an essentially weak structure must have far outweighed any conceivable advantage of concealment. The spines were slender and were constantly subject to fracture in battle or by accident and the animal must have expended no inconsiderable portion of its energies in repairing the broken structure.

There remains the suggestion that the spines are remnants of a formerly useful structure and their present condition is purely a physiological one due to overgrowth. It seems certain that when a structure has developed so far as to give an animal a great advantage it may continue to grow until it is rather a burden than a help. The structure starting as a protective feature may give the