182 GENERAL EVOLUTION. 



DEFINITIONS. 



a. The succession of construction of parts of a complex, was 

 originally a succession of identical repetitions ; and grade influ- 

 ence merely determined the number and location of such repeti- 

 tions. 



(i. Acceleration signifies addition to the number of those repeti- 

 tions during the period preceding maturity, as compared with the 

 preceding generation, and retardation signifies a reduction of the 

 numbers of such repetitions during the same time. 



y. The successive additions now characterizing the growth of the 

 highest animals are not exact repetitions of segments at this time, 

 because of influences brought to bear on cell-nutrition during long 

 periods. The nature of these influences is made the subject of 

 another section. 



In the endeavor to prove these positions, I will produce evi- 

 dence, first, that some simpler animals grow according to the prin- 

 ciple of modified repetitive addition, and that traces of it are to be 

 observed in the most complex ; second, that every addition to 

 structure which has resulted in the complexity of the higher ani- 

 mals was originally a repetition of a pre-existent structure. 



Detailed explanations of the law of repetitive addition are at- 

 tempted in the following pages, under two heads — segment-repeti- 

 tion, and cell-repetition. 



A. On Segment- Repetition. 



This is everywhere seen in the construction of animals and 

 plants. Double bilateral symmetry may serve as one example of 

 repetition in growth. 



a. Bilateral symmetry. Anatomists have little diSiculty in de- 

 termining the bilateral symmetry in most animals — that is, the 

 homologies of the parts on opposite sides of the median line. It 

 might be almost asserted that it was a necessity of organization ; 

 but, when we observe the growth of many plants, we are unde- 

 ceived. And though bilateral symmetry in the Ccelenterata and 

 many Articulata is perfect, yet in higher animals it is more or less 

 departed from. In the Vertebrata the Amphioxus is almost com- 

 pletely bilaterally symmetrical. In the fishes, the digestive system 

 is the only one which does not conform to it ; while in the birds 

 the reproductive system is atrophied on one side. In the ser- 

 pents the respiratory and part of the circulatory are similarly 



