62 PROBLEMS OF GENETICS 



little success. The problem is beset with difficulties as yet 

 insurmountable and of these one must be especially noticed. In 

 the living thing the process by which repetition and patterns 

 come into being consists partly in division but partly also in 

 growth. We have no means of studying the phenomena of 

 pattern-formation except in association with that of growth. 

 Growth soon ceases unless division takes place, and if growth is 

 impossible division soon ceases also. In consequence of this 

 fact that the final pattern is partly a product of growth, it can 

 never be used as unimpeachable evidence of the primary geo- 

 metrical relations of the members as laid down in the divisions. 



In the last chapter in referring to the problem of repetition 

 I introduced an analogy, comparing the patterns of the organic 

 world with those produced in unorganised materials by wave- 

 motion. In the preliminary stage of ignorance, having no more 

 trustworthy clue, I do not think it wholly unprofitable to consider 

 the applicability of this analogy somewhat more fully. It 

 possesses, as I hope to show, at least so much validity as to 

 encourage the belief that morphology may safely discard one 

 source of long-standing error and confusion. 



Those who have studied the structure of parts repeated in 

 series will have encountered the old morphological problem of 

 "Serial Homology," which has absorbed so much of the attention 

 of naturalists and especially of zoologists at various periods. 

 This problem includes two separate questions. The first of 

 these is the origin in evolution of the resemblance between two 

 organs occurring in a repeated series, of which the fore and hind 

 limbs of Vertebrates are the prerogative instance. From the 

 fact that these resemblances can be traced very far, often into 

 minute details of structure, many anatomists have inclined to 

 the opinion that the resemblance must originally have been still 

 more complete, and that the two limbs, for instance, must have 

 acquired their present forms by the differentiation of two iden- 

 tical groups of parts. 



Similar questions arise whenever parts are repeated in series, 

 whether the series be linear or radial, and, though less obviously, 

 even when the repetition is bilateral only. In each such example 



