112 THE PERMO-CARBONIFEROUS RED BEDS OF 



but where such a thing has happened the structures have the utmost utiH- 

 tarian value. A useless or only moderately useful structure would be one 

 of the quickest causes of extinction under such conditions. The author has 

 for many years sought diligently for an explanation from a utilitarian stand- 

 point for the spines of Dimetrodon, Clepsydrops, and Edaphosaurus, but he 

 has utterly failed either to imagine any use or to gain any suggestion from 

 others that would support such an explanation. 



Abel, in his Paleobiologie (p. 430) , cites a curious habit in the fishes Histio- 

 phorous gladius Brouss and Plagyodus ferox Lowe, which is substantiated by 

 several observers. Both have enormously enlarged dorsal fins, extending 

 nearly the whole length of the back and reaching a great height. They are 

 both accustomed to swim near the stixface, or to bask with the fins exposed, 

 and to drift before the wind. Although the fins are totally different in struc- 

 ture from those of Dimetrodon and Edaphosaurus, there is a decided similarity 

 in the external form, and here is a reminder of the original suggestion made 

 by Cope that both these animals were aquatic. In the opinion of the author, 

 it has been demonstrated that both Dimetrodon and Edaphosaurus were ter- 

 restrial, but he is so far from realizing any use for the dorsal fins of these 

 reptiles that he can not refrain from mentioning the only case which has come 

 to his notice where similar structures have an observed use. 



Dr. Alexander Ruthven has described to the author his observations on a 

 large specimen of Basiliscus, noted upon the banks of the Magdalena River, 

 in the United States of Colombia. This individual ran out upon a rock in 

 the river in ptirsuit of an insect and there encountered a strong breeze blow- 

 ing upon the side of the dorsal fin. Under the pressure of the wind the animal 

 was thrown well over to one side and seemed to have considerable difficulty 

 in maintaining its position. Such an effect would have been decidedly in- 

 creased upon animals with a fin proportionately much greater. This is but 

 another bit of evidence that the dorsal fins of Dimetrodon and Clepsydrops 

 were disadvantageous structures. 



(2) Environment, as the author has tried to show in another paper,'' is a 

 most complex conception, and can only be considered as the sum of all the 

 contacts of any organism with the world outside of itself, both organic and 

 inorganic. Isolation, as here used, conveys the idea of the freedom of the 

 animal from any repressive contacts. An organism might attain such a con- 

 dition in many conceivable ways, even though it were living in the midst of an 

 abundant life. If once such a condition were attained, structures which would 

 ordinarily be removed by selection in the struggle for existence would possibly 

 attain an unusual development and be eHminated only because of their effect 

 upon the animal which supported them, or cause the extinction of the animal. 



(3) This suggestion is a special case of the former. In the previous sug- 

 gestion we imagined the animal to be largely freed from any control of its 



" Case, Oecological factors of evolution, Bull. Wis. Soc. Nat. Hist., vol. 3, p. 169, 1905. 



