TRANSACTIONS OF SECTION D. 837 
Distortion of a curious kind is seen in cases of abrupt metamorphosis, where, 
as in the case of many Echinoderms, of Phoronis, and of the metabolic insects, the 
larva and the adult differ greatly in form, habits, mode of life, and very usually in 
the nature of their food and the mode of obtaining it; and the transition from one 
stage to the other is not a gradual but an abrupt one, at any rate so far as external 
characters are concerned. 
Sudden changes of this kind, as from the free swimming Pluteus to the 
creeping Echinus, or from the sluggish leaf-eating caterpillar to the dainty 
butterfly, cannot possibly be recapitulatory, for even if small jumps are permissible 
in nature, there is no room for bounds forward of this magnitude. Cases of 
abrupt metamorphosis may always be viewed as due to secondary modifications, 
and rarely, if ever, have any significance beyond the particular group of animals 
concerned. For example, a Pluteus larva may be recognised as belonging to the 
group of Echinoidea before the adult urchin has commenced to be formed within 
it, and the Lepidopteran caterpillar is already an unmistakable insect. Hence, 
for the explanation of the metamorphoses in these cases it is useless to look outside 
the groups of Echinoidea and Insecta respectively. 
Abrupt metamorphosis is always associated with great change in external form 
and appearance, and in mode of life, and very usually in mode of nutrition. A 
gradual transition in such cases is inadmissible, because in the intermediate stages 
the animal would be adapted to neither the larval nor the adult condition; a 
gradual conversion of the biting mouth parts of the caterpillar to the sucking 
proboscis of a moth would inevitably lead to starvation. The difficulty is evaded 
by retaining the external form and habits of one particular stage for an unduly 
long period, so that the relations of the animal to the surrounding environment 
remain unchanged, while internally preparations for the later stages are in pro- 
gress. Cinderella and the princess are equally possible entities, each being well 
adapted to her environment. The exigencies of the situation do not permit, how- 
ever, of a gradual change from one to the other: the transformation, at least as 
regards external appearance, must be abrupt. 
Kleinenberg has recently directed attention to cases in which the larval and 
adult organs develop independently; the larval nervous system, for instance, 
aborting completely and forming no part of that of the adult. Iam not sure that 
I fully understand Kleinenberg’s argument, but it seems very possible that such 
cases, which are probably far more numerous than is yet admitted, may be due to 
what may be termed the telescoping of ancestral stages one within another, which 
tales place in actual development, and may accordingly be grouped under the head 
of developmental convenience. Undue prolongation of an early ancestral stage, as 
in cases of abrupt metamorphosis, must involve modification, especially in the 
‘muscular and nervous systems ; in such cases a telescoping of ancestral stages takes 
place as we have seen, the adult being developed within the larva. Such tele- 
scoping must distort the recapitulatory history, and as the shape of the larva and 
adult may differ widely, an independent origin of organs, especially the muscular 
and nervous systems, may be acquired secondarily. 
The stage in the development of Squilla, in which the three posterior maxil- 
lipedes disappear completely, to reappear at a later stage in a totally different 
form, is not to be interpreted as meaning that the adult maxillipedes are entirely 
new structures unconnected historically with those of the larva. Neither is the 
annual shedding of the antlers of deer to be regarded as the repetition of an 
ancestral hornless condition intercalated historically between successive stages pro-~ 
vided with antlers. In both cases the explanation is afforded by convenience, 
whether of the embryo or adult. ; 
Many embryological modifications or distortions may be attributed to me- 
chanical causes, and may fairly be considered under the head of developmental 
conveniences. 
The amnion of higher vertebrates is a case in point, and is probably rightly 
explained as due in the first instance to sinking or depression of the embryo into 
the yolk, in order to avoid distortion through pressure against a hard unyielding 
eggshell. A similar device is employed, presumably for the same reason, in the 
