A418 LAMARCK, HIS LIFE AND WORK 
In the same line of study is Dr. R. T. Jackson’s* 
work on the mechanical origin of characters in the 
lamellibranch molluscs. ‘* The bivalve nature of the 
shell doubtless arose,’’ he says, “‘ from the splitting on 
the median line of a primitive univalvular ancestor;’’ 
and he adds: ‘* A parallel case is seen in the develop- 
ment of a bivalve shell in ancient crustaceans;’’ in 
both types of shells “‘ the form is induced by the 
mechanical conditions of the case.’” The adductor 
muscles of bivalve molluscs and crustaceans are, he 
shows plainly, the necessary consequence of the 
bivalvular condition. 
In his theory as to the origin of the siphon of the 
clam (MZya arenaria), he explains it in a manner 
identical with Lamarck’s explanations of the origin 
of the wading and swimming birds, etc., even to the 
use of the words “‘ effort ’’ and “‘ habit.”’ 
“In Mya arenaria we find a highly elongated 
siphon. In the young the siphon hardly extends 
beyond the borders of the valves, and then the ani- 
mal lives at or close to the surface. In progressive 
growth, as the animal burrows deeper, the siphon 
elongates, until it attains a length many times the 
total length of the valves. 
““ The ontogeny of the individual and the paleon- 
tology of the family both show that Mya came from 
a form with a very abbreviated siphon, and it seems 
evident that the long siphon of this genus was 
brought about by the effort to reach the surface 
induced by the habit of deep burial.’’ 
* «* Phylogeny of the Pelecypoda,”” Memoirs Boston Society Natural 
History, iv., 1890, pp. 277-400. Also, American Naturalist, 1891, 
XXV., pp. II-2I. 
