294 The American Geologist. May, ae 
e 
lingulas. Certainly in these cases there is no substantial or 
even approximate interference with the rostral extension of the 
shell. As many Lingulas are sand burrowers, the mechan- 
ical influence of a short or long pedicle does not seem under 
such circumstances evident. But assuming that a decreasing 
length of pedicle at last brings the beaks in contact or almost 
in contact with the surface of support, the tendency of growth 
at the beaks would seem to be repelled and a narrowing rather 
than widening effect produced, except as friction irritates the 
surface of the shell and might produce thickenings and callos- 
ity. 
The successive and related stages of hinge areas developed, 
and the succession of broader shells with cardinal growth, while 
accompanying a shorter and shorter pedicle until the organism 
may become sessile, can be as naturally assigned to a formal 
tendency, involved in the biological idea of a brachiopod, as to 
the varying accidents of position. In the Discinide where the 
shell is fixed by a peduncle passing through a hole in the ventral 
valve, and in the Craniide where there is no peduncle and the 
ventral valve is adherent, an opposite tendency has been started, 
and driven to its natural climax. These tendencies may be re- 
inforced by environment, mechanical conditions, strain, even 
survival or selection, but the organic fitness of the shell presup- 
poses an aptitude, which, aroused, pushes the shell towards lim- 
ital forms. 
The thought of formal tendency seems more forcibly sug- 
gested when we examine the outlines of development, prepared 
by Dr. Beecher, of the trilobite phylum. 
Here again Dr. Beecher significantly remarks: “Next must 
be considered the progressive addition of characters during 
the geological history of the protaspis, and in the ontogeny of 
the individual during its growth from the larval to the mature 
condition. It was shown in the paper already referred to, that 
there was an exact correlation to be made between the geolog- 
ical and zoological succession of first larval stages and adult 
forms, and therefore both may be reviewed together.” 
Broadly considered, the growth changes in the development 
or evolution of higher forms of trilobites from lower forms 
consists in a migration of the eyes from a ventral position to 
a dorsal position, the disappearance of the annulation in the 
me tes 
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