168 STUDIES IN EVOLUTION 



The others are more advanced stages of development, as 

 shown by the larger size of the individuals, and their hav- 

 ing characters which appear successively in the ontogeny 

 of a species belonging to the first order of development. 

 To attain the stage which is represented by actual speci- 

 mens, they must have passed through earlier stages, which 

 as yet have not been found. Furthermore, it is evident 

 that Barrande did not consider the orders after the first as 

 primitive, and characteristic of the genera cited, for, in 

 some remarks under the third order, he says : ^ "II est trfes- 

 vraisemblable, que la plupart des Trilobites de cette sec- 

 tion, si ce n'est tons, devront etre un jour transf^rds dans 

 la premiere, par suite de la ddcouverte probable d'embryons 

 sans segmens thoracique." 



The geological conditions necessary for the fossilization 

 of the minute larval forms of trilobites are such that only 

 in comparatively rare instances are any of the immature 

 stages preserved. Larval specimens are doubtless often over- 

 looked or neglected by collectors, but generally the sedi- 

 ments are too coarse for the preservation of these small and 

 delicate organisms. In certain horizons and rocks, how- 

 ever, such remains are quite abundant, and complete onto- 

 logical series may be obtained. Yet it is not strange that 

 series of equal completeness have not been found in all 

 Paleozoic horizons. 



The abbreviated or accelerated development of many of 

 the higher Crustacea has resulted in pushing the typical free- 

 swimming, larval nauplius so far forward in the ontogeny 

 that this stage is either eliminated or passed through while 

 the animal is still within the egg, so that when hatched it is 

 much advanced. Although the trilobites show distinct evi- 

 dence of accelerated development through the earlier inherit- 

 ance of certain characters which will be taken up later, yet 

 it is not believed that the normal series or periods of transfor- 

 mation were to any degree disturbed, since both the simf)lest 

 and most primitive genera whose ontogeny is known and the 

 most highly specialized forms agree in having a common 



