292 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



has taken to sonic means of sedentary defense, and in every case 

 the cost of upkeep has been too great and the group has gone under. 

 Every time the race has been to the swift, active, and strong, and 

 those that trusted in " passive resistance," in " defence and not de- 

 fiance," have gone under in competition with those that have been 

 prepared to face the risks involved in attack. The fact that turtles 

 and arniadiUos liave survived to the present indorses rather than 

 vitiates the principle. 



Other cases of rapid decline or sudden disappearance are more 

 diflicult to account for. The waning of the brachiopods, but not 

 yet their disappearance, the disappearance of the pteridosperms, the 

 rugose corals, the belemnoids and ammonoids synchronizing witli 

 the vanishing of many orders of reptiles, Avill long furnish subjects 

 for j-esearch by biologists and geologists. 



And it may well be that the explanation will often be along 

 biological rather than physical lines, such as those suggested for the 

 graptolites; Lapworth pointed out that their disappearance — in 

 spite of a brave effort of passive resistance — synchronized with the 

 great development of fishes, and the assumption by them of many 

 of the functions previously discharged by the trilobites. In other 

 cases the explanation may be more in the direction of that given for 

 the reptiles, to be referred to later. 



The rarity in the geological record of some of the stages in evolu- 

 tion, and the absence of others which must surely have existed, may 

 receive some explanation from what has frequently occurred in the 

 history of human iuAcntion. If variants arise and are subjected to 

 intense competition, they have no chance in the struggle for exist- 

 ence unless they show rapid improvement and development of the 

 favorable variations within a few generations. Hence the numbers 

 exhibiting each of the early stages of change will always be few and 

 the chances of their preservation slight. Those who have tried to 

 work out the stages in the history of an invention, for instance, will 

 appreciate the rarity of '' missing links " and the difHculty of filling 

 in every step toward the later perfection. These are looked upon as 

 " freaks " and, imlcss they present real and marked improvement, 

 are never manufactured on a large scale. Their numbers conse- 

 ([uently are few, and many of them are the victims of experiment 

 and often do not survive the experience. 



5. Perhaps the most wonderful result disclosed b}'^ a study of the 

 later part of the geological record is the steady and unbroken evo- 

 lution of brain from the earliest vertebrate animals to the present. 

 The exceeding slowness of the process in its early stages is not less 

 wonderful than its acceleration during the latest stages of geological 

 history. The disappearance of so many orders of reptiles at the end 



