THE METHOD OF CREATION OF ORGANIC FORMS.. 189 



humerus, second ulna, third and fourth carpals (tarsals), the last 

 phalanges, which are first specialized in this genus. 



By far the most usual modification is, however, complication bv 

 duplicated and triplicated and still more highly multiplied repeti- 

 tion in some segments of the archetype, and its omission in other 

 segments. Thus in the Tcenia, the cejihalic segments are much 

 modified, and the nature of the repetition might be thus ex- 

 pressed : Fig. 6 ; the simpler segments representing the body 

 segment, the two complex representing those of the head. In 

 each, it will be observed, the complication is represented by loops 

 of similar form, and each loop of dots which represent the cells in 

 the first linear (Fig. 1) arrangement. 



A somewhat similar figure might represent the nature of the 

 complication in the Myriapod. In the insect the additional com- 

 plications of the thoracic segments would alter the diagram near 

 the middle. 



In the vertebrate cranium a somewhat similar diagram might 

 be used, except that the modification of the segments or vertebrae, 

 as compared with the segments of the vertebral column, is not by 

 rciDctition with modification of the parts of each segment, but 

 rather by modification of the forms of the parts of the segments. 

 The basicranial segments thus compare with the dorsal vertebrae 

 as the segments of the limb of Plesiosaurus do with those of Ich- 

 thyosaurus. 



The above considerations have reference to repetition of parts 

 in a linear direction. Centrifugal repetition is seen in the addi- 

 tion of chambers to the heart, by the subdivision in the earliest 

 stages into auricle and ventricle in the linear direction, con- 

 sidered in connection with the earlier division of each in the 

 transverse direction by the growth of partitions. This mode of 

 repetitive addition is not readily represented by diagram. 



A good example of repetitive addition, in both linear and 

 transverse direction, may be found in the successive complication 

 of tooth structure seen in Mammalia. In the dolphin, the dental 

 series may be represented thus : Fig. 7 ; in the squalodon thus : 

 Fig. 8 ; in the cat : Fig. 9 ; in the dog : Fig. 10 : in man : Fig. 

 11 ; in some Insectivora : Fig. 12. 



The circles represented here are each a simple cusp. 



In conclusion, the directions of repetitive growth may be 

 tabulated as follows : The types to the left represent the original ; 

 to the right, the derivative : 



