SEGMENTATION 67 



In various and more complex aspects the same question is 

 debated in regard to the cranial and spinal nerves, the branches 

 of the aorta, the appendages of Arthropoda, and indeed in re- 

 gard to all such series of differentiated parts in linear or suc- 

 cessive repetition. Persons exercised with these problems 

 should before making up their minds consider how similar 

 questions would be answered in the case of any series of rhyth- 

 mical repetitions formed by mechanical agencies. In the case 

 of our illustration of the ripples in the sand, given the same forces 

 acting on the same materials in the same area, the number of 

 ripples produced will be the same, and the nth ripple counting 

 from the end of the series will stand in the same place whenever 

 the series is evoked. If any of the conditions be changed, the 

 number and shapes can be changed too, and a fresh "distribution 

 of differentiation" created. Stated in this form it is evident 

 that the considerations which would guide the judgment in the 

 case of the sand ripples are not essentially different from those 

 which govern the problem of individual homology in its applica- 

 tion to vertebrae, nerves, or digits. 



The fact that the unit of repetition is also the unit of growth 

 is the source of the obscurity which veils the process. When we 

 compare the skeleton of a long-tailed monkey with that of a 

 short-tailed or tailless ape we see at once how readily the addi- 

 tional series of caudal segments may be described as a conse- 

 quence of the propagation of the "waves" of segmentation 

 beyond the point where they die out in the shorter column, and 

 we see that with an extension of the series of repetitions there is 

 growth and extension of material. 



The considerations which apply to this example will be found 

 operating in many cases of the variation of terminal members of 

 linear series. Some of these series, like the teeth of the dog, 

 end in a terminal member of a size greatly reduced below that of 

 the next to it. Even when there is thus a definite specialisation 

 of the last member of the series it not infrequently happens that 

 the addition, by variation, of a member beyond the normal 

 terminal, is accompanied by a very palpable increase in size of 

 the member which stands numerically in the place of the normal 



