ALPHEUS HYATT 



L xil 



accepted as a recapitulation of the phytogeny of the 

 group in hand. Stages are not equally clear in all types, 

 for stages may be skipped, or may be telescoped in 

 specialized forms, but in primitive forms {Nautilus, 

 Lingula, Pecten, Cidaris) they are astonishingly clear 

 and complete. By some investigators stages have been 

 denied and the recapitulation theory considered a myth. 

 I can not enter into discussion here, but can simply say 

 that it is felt that opponents have not considered the 

 evidence. Cumings 6 has recently put the matter well 

 in a defence of the Eecapitulation Theory. . 



The principle of acceleration of development, origi- 

 nated by Professor Hyatt, 7 is at once an explanation of 

 the existence of stages in development, and the loss, or 

 skipping of stages as well. This principle maintains 

 that features appearing at or near the adult period of 

 development are inherited at earlier and earlier stages 

 in successive generations until they exist only in the 

 extreme young or are skipped as stages in development. 



As examples of accelerations: In certain Palaeozoic 

 Echini the full number of columns of ambulacral and 

 interambulacral plates are attained only in the adult. 

 In more specialized species the similar columns are taken 

 on much earlier in both areas than they appear in lower 

 species (Melonecliinus). The pelecypod Hinnites is at- 

 tached by the fixation of one valve to foreign objects 

 when about one fourth grown, and then loses its young 

 pecteniform character. The allied Spondiilus is attached 

 when very much younger and thus earlier loses the 

 similar stage. Plicatula is attached at the close of the 

 prodissoconch stage and has lost the pecteniform stage 

 altogether. In primitive tritobites (Solenopleura, Sao) 

 the protaspis is rounded with neither dorsal eyes nor 



•E. R. Cumings, "Palaeontology and the Recapitulation Theory," PfOA 

 Indiana Acad. Sci., twenty-fifth anniversary meeting, 1909. 



T A. Hyatt, "On Parallelism between the Different Stages of Life in the 

 Individual and Those in the Entire Group of the Molluscous Order Tetra- 

 branebiata," Mem. Boston Soc. Nat. JH.it.. V I. 1. 1^'<'. p- _- f ' 3 - s, ' e ^ ,e ° 



Vol. 10, pp. 302-3o1?. 



