NORTH AMERICA AND THEIR VERTEBRATE FAUNA. II5 



Though these remarks apply to the Amphipods Gammarus and Allor- 

 chestes, and other invertebrates, it is apparent that they are just as appHcable 

 to the vertebrate Dimetrodon and related forms among the Pelycosauria. 

 Practically withdrawn from any struggle for existence (so far as we can see) 

 and abundantly nourished, the excess vitality of the animal was used in the 

 elaboration of structures, possibly useful in their inception, until they became 

 elements of weakness, and finally an efficient cause for the extinction of 

 the group. The drain upon the animals' vitality necessary to elaborate such 

 structures and to maintain them in repair, for they were frequently broken 

 by accident and in conflict with other animals, could only be supported when 

 the animal was in the height of its vigor. When for any reason the animals' 

 powers were lessened, the support of the structure would become a disad- 

 vantage so great that it would result in the decadence of the individual and 

 the extinction of the group. 



Loomis'^ has argued for a "momentum of variation," or evolution, which 

 he supports by many cases very similar to that of Dimetrodon. If such a 

 principle exists, as seems very probable, the tendency to maintain structures 

 already developed would be an added cause for the extinction of a group when 

 it had commenced its decadence. 



This argument is based on the belief that the spines were useless struc- 

 tures. Jaekel and Abel have contended that they were of distinct importance 

 in repelling the attacks of enemies, but, as has been shown, the only enemies 

 were individuals of the same genus, and a struggle between individuals of a 

 genus could have no bearing on the development of the genus itself. 



A similar argument can not be extended to the genera Edaplwsaurits, 

 Diplocaulus, or Platyhistrix. They may have developed their peculiar struc- 

 tures under conditions outlined in the .second suggestion. 



The line of reasoning here followed leads inevitably to the question 

 whether the perfect adaptation of any group to its environment may not be 

 as great a danger to the group as a severe struggle for existence. In the 

 struggle the individual suffers, but the group represented by the best indi- 

 viduals survives. If the group as a whole reaches, or approximates, perfect 

 adaptation, the momentum of variation, aided by superabundant vitality 

 due to full nourishment and lack of effective control, would result in the exag- 

 geration of structures until they became a fatal defect. Perfect adaptation 

 is a possible cause of extinction. 



(4) The last one of the possible causes for the development of seemingly 

 useless structures is the disturbance of the phylum due to approaching extinc- 

 tion, as suggested by Beecher.'' When the high-spined forms were supposed 

 to be the dominant types of the Permo-Carboniferous vertebrate fauna and 

 their distribution was supposed to be much more limited than it is now known 

 to be, this idea would have been applied to the whole fauna, but now it can 



" Loomis, Amer. Nat., vol. 39, p. 839, 1905. 

 '' Beecher, Amcr. Jour. Sci., vol. vii, 1898. 



