548 WILLIAM BATESON. 
water-vessel of Echinodermata, 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 differentiated 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, &. 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 differen- 
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 Hydromedusz, the division of the 
ambulacra of Echinoderms into bivium and trivium cul- 
minating in the bilateral symmetry of Holothurians, differen- 
tiation between vertebre, &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 in 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 
