96 PROF. W. GARSTAiJa ON THE THEORY OF RECAPITULATION : 
Now note the discrepancies. Except possibly in the anal interradius, 
there is no trace of praj-Cambrian irregularity in the number and arrange- 
ment of the skeletal plates, of the earlier acquisition of radial sj'mmetry by 
the ambulacra and its later imposition upon the plates of the calyx; no sign 
of the derivation of the stalk by constriction of a pyriform base ; no evidence 
of the oral plates having originally formed a solid disk, above the sutures of 
which the ambulacra ran. Except for certain additional dislocations to be 
referred to in a moment, it is just as in the development of the skeleton of a 
Vertebrate limb : the number of the skeletal elements is fixed from the 
beginning (even the pattern of the stem-joints) and ontogeny reveals no signs 
of their past history — with two exceptions : the migrations of the Anal plate 
and the composition of the Centro-dorsal. The former is a precious record 
of the change exhibited by the ancestral Dendrocrinoids, when, as arm- 
structure changed and flexibility increased, the diminishing anal chimney 
(fig. iii.) no longer required a buttress in the calyx wall to support it. But 
its retention in the ontogeny of Antedon is no proof of the normality of so 
precise a record of ancestral change : rather is it the exception which proves 
the rule of absence of such records. It is comparable with the " useless "" 
iiotochord of the Vertebrate embryo. It has no part to play in the adult,, 
because, as growth proceeds, the bases of the arms take over the main support 
of the body; but in the larva the patina is the sole support, and, as an Anal 
plate (and a Radianal as well) was a constant inherited element of the cup. 
through nearly the whole of Palaeozoic time, it is scarcely surprising that it 
should be retained in that part of the life-cycle where it is still conceivably 
useful. On the other hand, it is sT^ueezed out of the cup as soon as the anal 
tube, by remaining small, withdraws any demand for its retention, and when 
the flexible incorporation of the five arm-bases in the cup sets up a counter- 
demand for strict peutameral symmetry. This demand, so far as the Radianal 
is concerne'd, has long since been met by the complete elimination of 
that plate from the ontogeny. " One thing at a time " is nature's rule. 
Similarly the ontogenetic history of the Centrodorsal is a physical necessity 
if one plate is to be made by the amalgamation of a number ((•/'. development 
of vertebrse, pore-plates of Echinus, &c.) ; and the other recapitulative 
features of the ontogeny (arm-development &c.) are examples of other 
necessities of differentiation, since you cannot get 2, except by duplicating 1. 
Adult recapitulation demands that the arm-branches should extend to the 
full length of the arms (as in Cyathocrinoids) before they reduce themselves 
alternately to the dimensions of pinnules. This they do not do. They take 
the shortest route to their goal, so far as constitution, not ancestry, will 
allow them. 
But these discrepancies with phylogeny are trifles beside the phenomena 
of development of the Oral and Eadial plates. In the whole series of adult 
ancestors from Cambrian times to the present, not one possessed Oral plates 
