1 76 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



A number of illustrations of the parallelism be- 

 tween taxonomy, ontogeny, and phylogeny may now 

 be given. 



i. PARALLELISM IN THE BRACHIOPODA. 



For the following abstract I am indebted to Mr. C. 

 E. Beecher of New Haven, whose excellent work in 

 this field is well known. 



The parallelism between the ontogeny and phy- 

 logeny in the Brachiopoda has been worked out in 

 numerous instances. 1 To illustrate these, some more or 

 less familiar genera may be taken as characteristic ex- 

 amples. 



Lingula has been shown by Hall and Clarke (Pa/. 

 New York, Vol. VIII., 1892) to have had its inception 

 in the Ordovician. In the ontogeny of both recent and 

 fossil forms, the first shelled stage has a straight hinge 

 line, nearly equal in length to the width of the shell. 

 This stage may be correlated with the more ancient 

 genus Paterina from the lowest Cambrian. Subsequent 

 growth produces a form resembling Obolella, a Cam- 

 brian and Ordovician genus. Then the linguloid type 

 of structure appears at an adolescent period, and is 

 completed at maturity. Thus, Lingula has ontogenetic 

 stages corresponding to (i) Paterina, (2) Obolella, and 

 (3) Lingula, of which the first two occur as adult forms 

 in geological formations older than any known Lin- 

 gula. 



Paterina represents the radicle of the brachiopods. 



1C. E. Beecher, "Development of the Brachiopoda," Part I., Introduc- 

 tion, Amcr. Journ, Sci., Vol, XLL, April, 1891 ; " Development of the Brachio- 

 poda," Part II., Classification of the Stages of Growth and Decline, Amer. 

 Jour. Sci., Vol. XLIV., August, 1892 ; "Development of Bilobites," Amer. 

 Jour. Sci., Vol. XLII., July, 1891 ; " Revision of the Families of Loop-bearing 

 Brachiopoda," Trans. Conn. Acad. Sci., Vol. IX., May, 1893. 



