LAWS OF ORGANIC DEVELOPMENT. 597 
Example 3.—Many individuals of Triodopsis tridentata from 
eastern North Carolina occur without the lip-teeth characteristic of 
the genus Triodopsis. Hence these specimens, though of common 
origin with others of the species, must be referred to another genus. 
Example 4.— Structural characters are known in many, if not all, 
species which are said to be “ inconstant,” being present or absent 
_indifferently, thus being useless for definition. They may be 
rudimental when present or considerably developed. The pres- 
ence or absence of wings in some species of insects may be cited ; 
also the presence of generic characters in the male sex of many 
Coleoptera and their absence in the females. The characters of 
males, females, workers and soldiers in bees and ants may be 
added. All these facts belong to the same category as those cited 
among deer and mollusks and have a similar explanation. 
Example 5. does not seem to be the law in ‘‘ retardation” 
that parallelisms exhibited by the series in its rise to its highest 
point of development should retrace the steps by which it attained 
it, and that ‘‘exact parallelisms” should be exhibited in a reversed 
order. Parallelisms, it is true, are exhibited ; but so far as I have 
observed always “inexact,” often in a high degree. A marked 
case of retardation occurs in the dental development of a number 
of persons who have come under my observation in the neighbor- 
hood of Philadelphia. It is not very uncommon to find persons in 
whom the third molars in both jaws are incomplete as to number, 
one, two, three, or all, being deficient. It is still more common 
for them to be incompletely covered by the enamel layer, and to 
become in consequence so worthless as to require early removal. 
I am acquainted with two families in which the absence of the 
exterior upper incisor on each side is common. In one of these 
the second and third generation have inherited it from the mother’s 
side, and it now characterizes many of the children. The signifi- 
cance of this modification will be best understood by examining 
the dental structures of the Quadrumana in general, commencing 
with the highest family and the modification, we have :— 
3 
g 
a 
© 
ë 
3 
F 
F 
z 
J 
. 
cola caka go'co caco coco cathy 
WINS hn 
. 
Ppa Pat pa pelt Paid tps 1ifat 
