66 PROBLEMS OF GENETICS 



that these differences which correspond with the differenti- 

 ations of the organic series are due to special heterogeneity in 

 the conditions or in the materials, and it never occurs to us to 

 suppose that all the members must have been primordially alike. 

 For example, in the case of ripple-marks on the sand, which I 

 choose as one of the most familiar and obvious illustrations 

 of a repeated series due to mechanical agencies, if we notice 

 one ripple different in form from those adjacent to it, we do 

 not suppose that this variation must have been brought about by 

 deformation of a ripple which was at first formed like the others, 

 but we ascribe it to a difference in the sand at that point, or to a 

 difference in the way in which the wind or the tide dealt with it. 

 We may press the analogy further by observing that in as much 

 as such a series of waves has a beginning and an end, it possesses 

 polarity like that of the various linear series of parts in organisms, 

 and even the formation of each member must influence the 

 shape of its successor. Since in an organism the beginning and 

 end of the series are always included, some differentiation among 

 the repetitions must be inevitable. If therefore it be conceded, 

 as I think it must, that segmentation and pattern are the con- 

 sequence of a periodic process we realize that it is at least as 

 easy to imagine the formation of such a series of parts having 

 family likeness combined with differentiation as it would be to 

 conceive of their arising primordially as a series of identical repe- 

 titions. The suggestion that the likenesses which we now per- 

 ceive are the remains of a still more complete resemblance 

 is a substitution of a more complex conception for a simpler one. 

 The other question raised by the problem of Serial Homology 

 is how far there is a correspondence between individual members 

 of series when the series differ from each other either in the 

 number of parts, or in the mode of distribution of differentiation 

 among them. Students, for example, of vertebrate morphology 

 debate whether the nth vertebra which carries the pelvic girdle 

 in Lizard A is individually homologous with the n+xth vertebra 

 which fulfils this function in Lizard B,or whether it is not more 

 truly homologous with the vertebra standing in the nth ordinal 

 position, though that vertebra in Lizard B is free. 



