4i8 



LAMARCK, HIS LIFE AND WOKK 



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 {Mya 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 thePelecypoda," Memoirs Boston Society Natural 

 History, iv. , l8go, pp. 277-400. Also, American Naturalist, 1891, 

 XXV., pp. 1 1-2 1. 



