296 The American Geologist. ae 
wards change in a definite direction, taken in conjunction with 
external influences which act chiefly by way of accelerating 
or retarding the process of change, and in relation with indi- 
vidual differences of susceptibility to stimulus.” 
Formal tendency is evident in less intricate and more 
easily verifiable cases amongst fossils. Dr. Beecher whose re 
searches are so comprehensive has pointed out in his essay 
upon the “Origin and Significance of Spines’”’ that there is a 
“formal tendency” whereby in organisms there is “the smootii 
rounded embryo or larval form progressively acquiring more 
and more pronounced and highly differentiated characters 
through youth and maturity,” and although this growth has 
been analyzed by him as referable to eleven categories of sec- 
ondary influence, it is impossible to reserve the conviction that 
it embodies something more innate. 
In fossils there are tendencies in form which seem quite 
inexplicable on the assumption of purely secondary causes, as 
the increasing ventro-dorsal convexity, of some groups of 
Rhynchonella, the rostral elongation of others (Rhynchotreta), 
the extension of the hinge line in Spirifer, the widening of the 
cardinal area, also in Spirifer, the advancing arcuateness of the 
ventral valves in Stropheodonta, etc., while in internal struc- 
ture the progressive modifications of the internal loop form . 
series of related states apparently independent of any con- 
ceivable external circumstances. 
In the Orthidz the development of a reverted pedicle valve 
which might be said to begin in such forms as O. flabellites, O. 
tricenaria, continues until such extravagant distortion is 
reached as is displayed in O. (Plesiomys) porcata, O. retrorsa, 
Plectorthis kankakensis. The tendency to bend back the ven- 
tral valve may have originated in some accommodation to po- 
sition, but once started it has progressed to abnormal limits, 
by an organic necessity. 
The extreme convexity nodular development and _ resupi- 
nate exaggeration in Derbya seems similarly to be rather an 
organic climax to the same anticipatory features in Streptor- 
hynchus, Orthotetes, and Meekella. 
The arched ventral valve in Strophonella, Stropheodonta 
becomes hemispherical in such advanced forms of the same 
group as S. hemispherica, S. concava and Productus costatus. 
. 
