THE METHOD OF CREATION OF ORGANIC FORMS. I79 



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. — It 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 'Mnexact," 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 ex- 

 terior 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 Qiiadrumana in general. Commencing 

 with the highest family and its abnormal dentition, w^e have : 



Incisors. Canines. Premolars. Molars. 



rr ■ '1 ( Abnormal. i i" f a — 2 



( >iiormal. f 1 t t 



Simiidce f \ I 3 



Cebidce .... f "i- 3 f 



LemuridcB f \ I — 3 3 



Mammalia^ Normal | \ 4 I 



In this table we see a decline in the number of teeth of the 

 higher groups. Thus, the premolars are one less than the normal 

 number in the whole order, and they lose one in each jaw in tlie 

 Old World apes, and man. The molars maintain the normal num- 

 ber throughout, but the third in both jaws is in the SimiidcB 

 reduced by the loss of a fifth or odd tubercle, thus becoming four- 

 lobed. In the upper jaw, this is first lost in the Semnopithecus ; 

 in the lower, in the next highest genus Cercopithecus. In Homo 

 its appearance is "retarded," the interval between that event and 

 the protrusion of the second molar— six to ten years— being rela- 

 tively greater than in any genus of Qiiadrumana. Its absence is 



