278 ROBERT TRACY JACKSON ON THE 



For the freest opportunity to study collections under their charge and for frequent as- 

 sistance while studying those collections ray obligations are due to Dr. C. A. White and 

 Dr. William II. Dall of Washington; Professor Angelo Ileilprin of Philadelphia; Pro- 

 fessor R. P. Whitfield of IN^ew York; Dr. J.Walter Fevvkes and Mr. Samuel Henshaw. 

 For material borrowed for study, I am indebted to Dr. White, Dr. Dall, the Museum of 

 Comparative Zoology, the State Museum of New York and the Boston Society of Nat- 

 ural History. I have to thank Colonel Macdonald, United States Commissioner of 

 Fisheries, for the electrotypes of figures of Ostrea edidis and Professor A. E. Verrill 

 for electrotypes of Vermetus, Pecten and Anomia. 



In order to unravel with any reasonable hope of success the relationships of the mul- 

 titude of fossil forms of animal life in which only the hard parts are commonly preserved, 

 and to demonstrate the connection between the living and fossil representatives of allied 

 groups, it is most important to study with minute care the hard parts in living genera. 

 It is necessary to know as far as possible the structure, development and functional ex- 

 pression of the hard parts, both by themselves and in their relation to the soft parts. To 

 attain this end, studies of any given animal should be made on all stages of growth from 

 the embryonic upward, and in so far as the young is considered as a living expression 

 of the adults of ancestral groups, this period of the life-history must be studied with special 

 care. 



On the value of anatomy and embryology to the paleontologist studying the affinities 

 of groups I would quote the following passage from Professor Hixxley's Anatomy of In- 

 vertebrated Animals, p. 587. " Already indications are not wanting that the vast multi- 

 tude of fossil Arthropods, Mollusks, Echinoderms and Zoophytes, now known, will yield 

 satisfactory evidence of the filiation of successive forms, when the investigations of palae- 

 ontologists ai-e not merely actuated by the desire to discover time-marks and to multiply 

 species, but are guided by that perception of the importance of morphological facts which 

 can only be conferred by a large and thorough acquaintance with anatomy and erabr}^- 

 ology. But, under this aspect, the palaeontology of the Invertebrata has yet to be cre- 

 ated." 



The results of my investigations give abundant proofs that a study of the soft parts 

 and especially studies of life habits and environing influences will throw light on the 

 meaning of the form of hard parts. They all lead toward the conclusion that external 

 hard parts are built conforming to the needs of the animal and are modified by circum- 

 stances of environment. Many facts discussed in this paper are directly opposed to the 

 assumption that hard parts are of prescribed form, to which the animal adapts itself, as I 

 have found most active response in the form of hard pai'ts to changed or changing con- 

 ditions of the anatomy or habits of the animal. The changes in form thus introduced 

 might well be called cases of "mechanical genesis," adopting Professor Ryder's apt phrase 

 used of the development of toothed forms in Vertebrates.^ When the meaning of feat- 

 ures of shell form in living animals is ascertained, it is recognized that they make im- 

 portant factors in the consideration of fossils and the serial connection of groups. 



To unite living and fossil foi'ms of a group of Pelecypoda in a common study is the 



' Facts bearing on tliis point ai'C especially considered andiu the discussion of Perna (section viii), Pecten (see- 



in the origin of tlic ostrean form of sliell (section vil), tioiis ix aiul x), and Anomia (section xii). 



