80 WILLIAM BATESON. 



water- vessel of Ecliinodermata, that some of the repetitions are 

 presented early in the development. 



Besides the probability that most repetitions occur in the 

 first instance in adults, or, at least, in mature individuals, it 

 may also be noted as a general feature of them that they are at 

 first very similar to, if not identical with, each other. For on 

 their first appearance in an individual they do not generally 

 arise phylogenetically in the condition which may be supposed 

 to have been that in which the original organs of the same 

 series first arose, but rather from the first they are found as 

 fully difi'erentiated copies of the other members of the series, 

 and not as rudiments. For example, the horns and teeth of 

 mammals, whose number varies greatly, are, in those forms 

 which possess additional ones, not repeated as tubercles or as 

 plates, but rather as fully developed horns, teeth, &c. Though 

 this is not universally true it is yet sufficiently well marked a 

 feature to be of great importance in estimating the probability 

 of the recurrence of such a complicated organ as a vertebra 

 with its correlated parts within narrow limits of race. But no 

 less noticeable is the tendency towards a subsequent diff'eren- 

 tiation and division of function among members of a series of 

 similar parts as soon as the series is formed or any new 

 member is added to it. This is of course to be seen in the 

 case of the tentacles of Hydromedusse, the division of the 

 ambulacra of Echinoderms into bivium and trivium cul- 

 fliinating in the bilateral symmetry of Holothurians, diff"eren- 

 tiation between vertebrae, &c. 



Beyond this little can be predicated of the mode of occur- 

 rence of repetition of parts. Nothing is attained by analysis 

 of the known facts which can be felt to be iu any way a basis 

 from which to interpret them. This much alone is clear, that 

 the meaning of cases of complex repetition will not be found 

 in the search for an ancestral form, which, itself presenting 

 this same character, may be twisted into a representation of 

 its supposed descendant. Such forms there may be, but in 

 finding them the real problem is not even resolved a single 

 stage ; for from whence was their repetition derived ? The 



