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 Ostreide, 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. 
