INSTITUTE OF SCIEN'CE, PHILADELPHIA. 497 



against the walls of the burrow less liable to slip by one another at the dorsal 

 edge, and thus reduce the effectiveness of the animal's grasp of them (if I 

 may use such an expression). The energetic action of the foot and pedal 

 muscles when at work, and the feeble character of the ligament in this genus, 

 would render such an accident likely if the valves where pressed against one 

 another had merely a thin sharp edge. Their reflection gives a broader bear- 

 ing surface and makes the hinge more secure. The fact that the anterior 

 adductor is so much more nearly dorsal in its situation than the posterior, 

 might possibly even give it a partially opposite pull, which, if true, would be 

 all that would be needed to start the muscle, pushed by natural selection, on 

 its dorsal journey. These hypotheses can be best understood by reading them 

 with the specimens in hand, and I would ask any doubting readers to go over 

 the matter with the shells before them. When once the dynamic influence of 

 action and environment are made clear in a single instance, one can there- 

 after hardly study any feature of the shell without having those influences 

 ill mind. The result is a flood of light on many points which have hitherto 

 been accepted as mere inert facts. Given a full knowledge of the animal's 

 motions, efforts and surroundings, with its immediate pedigree, and I am fully 

 convinced there is no feature of its more external and operative organs and 

 structure which cannot be accounted for by a reasonable hypothesis, based on 

 dynamic principles, promoted by natural selection. The origin of differentia- 

 tion is the impact of forces, which drive, in each case, not all ways, but in one 

 general direction ; wliich, having been taken, is narrowed to the most desirable 

 path by natural selection. In this way, aided by heredity, all species, genera 

 and groups of every sort have arisen. In given circumstances there may have 

 been more than one path out of the melee, but the choice of paths in every 

 case must have been determined by the preponderating dynamic forces of each 

 particular case. Many superficial characters, especially color, are often the 

 direct result of food or other factors of the environment, but modifications of 

 the hard parts concerned in motion, bones, scales, shells, etc., invariably pro- 

 ceed, as far as my own observation indicates, from a cooperation of force, 

 natural selection and heredity. 



The Ligament and Resilium. — Embryology shows, according to the 

 observations of Lankester and others, that the shell gland of embryonic Pelecy- 

 pods is a single organ, somewhat saddleshaped, which crosses the whole region 

 which in the adult is umbonal, and from which the paired valves are subse- 

 quently developed. These valves (in Pisidiiini) are originally somewhat dis- 

 tant from one another and approximate with growth. The shelly matter of 

 the valves in all Pelecypods is deposited upon a horny layer which protects 

 the exterior from attack by acids in the water, wear, etc. ; and it is generally 

 admitted that this layer, called the epidermis or periostracum, is continuous in 

 the embryo and that that portion of it lying between the prodissoconchs (em- 

 bryonic valves) is modified and developed to form the elastic link which 



