BIOLOGICAL INVESTIGATIONS. 131 



like form recapitulated in every ovum more or less ; but leaving 

 this remote region on one side, we find the initial point for 

 any branch, large or small, of the animal kingdom is very often 

 a primitive form, acquiring the characters of its successors late 

 in life ; or they may have them as yet in such indefinite shape 

 that they become perceptible only after this primitive ancestor 

 has been traced to better characterized descendants through 

 the gradations of intermediate forms. Several phyla have 

 been followed out by the painful and tedious process of track- 

 ing from species to species, and others by using the ontogeny 

 or life histories of different forms as indices of the natural 

 relations of groups. ^ 



1 The following is a partial list of these : 



Beecher. Rev. of the Families of Loop-bearing Brachiopoda. Trans. Conn. 



Acad. Sci. Vol. iii, 4th Ser., 1893, pp. 376-391. 

 Beecher. Outline of a Natural Classification of the Trilobites. Amer. Journ. 



of Sci. Vol. iii, 4th Sen, 1S97, pp. 86-106 and 181-207. 

 Beecher. Trilobites in ZittePs Text-Book of Paleontology, Am. Ed. 

 BUCKMAN, S. S. Mon. Inf. Ool. Ammonites. Paleontogr. Soc. London, 1887, 



and still in course of publication. 

 HiLGENDORF. Planorbis multiformis in Steinh. Siisswasserkalk. Monatab. der 



Preuss. Acad, der IViss., p. 474. Berlin, July, 1866. 

 Hyatt. Genesis of Tertiary species of Planorbis at Steinheim. Ann. Mem. Post. 



Soc. Nat. Hist. 1880. 

 Hyatt. Genesis of the Arietidas. Smith's Contr.l No. 673, 1889, and Me?n. 



Mils. Comp. Zodl. 

 Hyatt. Genera of Fossil Cephalopoda. Proc. Post. Soc. Nat. Hist. Vol. xxii, 



1S83, pp. 253-338. 

 Hyatt. Phylogeny of an Acquired Characteristic. Proc. Amer. Phil. Soc, 



Philadelphia. Vol. xxxii. No. 143, 1894. 

 Hyatt. Cephalopoda in ZittePs Text-Book of Paleontology, Am. Ed., in course of 



publication. 

 Jackson. Phylogeny of Pelecypoda. Mem. Bost. Soc. Nat. Hist. Vol. iv, No. 8, 



1890, pp. 277-400. 

 Jackson. Studies of Palechinoidea. Bidl. Geol. Soc. Amer. Vol. vii, pp. 



171-254. 

 Neumayer. Beit. z. Kennt. foss. Binnen-Fauna. fahrb. Geol. Reichs. Bd. xix. 



No. 3, 1869, pp. 355-382. 

 SCHUCHERT. A Classification of the Brachiopoda. Amer. Geol. Vols, xi and 



xiii, 1893-94. 

 ScHUCHERT. Brachiopoda in ZitteVsText-Book of Paleontology. Vol. i, Pt. I, 



1896. 

 WuRTENBERGER. Studien liber die Stammesgeschichte der Ammoniten. Dar- 

 winistische Schriften. No. 5. Leipzig, 1880. 



