420 LAMARCK, HIS LIFE AND WORK 



attached valve is the most highly modified, and the 

 free is least modified, retaining more fully ancestral 

 characters. Therefore, it is to the free young before 

 fixation takes place and to the free, least-modified 

 valve that we must turn in tracing genetic relations 

 of attached groups. Another characteristic of at- 

 tached pelecypods is camerated structure, which is 

 most frequent and extensive in the thick attached 

 valve. The form as above described is characteris- 

 tic of the Ostreidas, Hinnites, Spondylus, and Plica- 

 tula, Dimya, Pernostrea, Aetheria, and Mulleria; 

 and Chama and its near allies. These various gen- 

 era, though ostreiform in the adult, are equivalvular 

 and of totally different form in the free young. The 

 several types cited are from widely separated fam- 

 ilies of pelecypods, yet all, under the same given 

 conditions, adopt a closely similar form, which is 

 strong proof that common forces acting on all alike 

 have induced the resulting form. What the forces 

 are that have induced this form it is not easy to see 

 from the study of this form alone ; but the ostrean 

 form is the base of a series, from the summit of 

 which we get a clearer view." (Amer. Nat., pp. 

 18-20.) 



Here we see, plainly brought out by Jackson's re- 

 searches, that the Lamarckian factors of change of 

 environment and consequently of habit, effort, use 

 and disuse, or mechanical strains resulting in the 

 modifications of some, and even the appearance of 

 new organs, as the adductor muscles, have originated 

 new characters which are peculiar to the class, and 

 thus a new class has been originated. The mollusca, 

 indeed, show to an unusual extent the influence of 

 a change in environment and of use and disuse in the 

 formation of classes. 



