90 SECTIONAL ADDRESSES. 



surely somewhiere over the wide earth we ought to have found a 

 Zygobranchiate snail with its torsion incomplete. There are, I believe, 

 180 degrees in a half-circle. Allowing 10 degrees as a reasonable range 

 for each successive stable position in a series of adult modifications, 

 we have eighteen different positions in which some snail or other might 

 reasonably be expected to have made a halt in the orthogenetic advance. 



In the Opisthobranchiate MoUusca there is abundant evidence that an 

 evolutional process of detorsion has actually occurred. Anus, gill, and 

 kidney, at first in their mantle-chamber, have travelled back again along 

 the right side of the body, reversing the original order of events. There 

 can be no confusion between the stages of retreat and those of advance 

 because all of these unwinding snails have come back without certain 

 organs of the original left side with which they went forward. Every 

 possible stage from complete torsion through partial to complete detorsion 

 — far more than the eighteen grades which I assumed — is represented 

 to-day by families, genera, and countless species — Actceon, Bulla, Philine, 

 Aplysia, Eolis, Doris, &c., not to speak of the Pteropods derived from them. 

 Their variety shows us what must have occurred on the forward march 

 of the prse-Gastropods if it proceeded by comparable stages ; and although 

 many that went forward would certainly fall out in the long lapse of time 

 owing to changed conditions, yet there must have been opportunities for 

 adaptations capable of preserving the essentials of one or more of these 

 eighteen advancing types. We know this from facts. In each minor 

 group of primitive snails, possessing clear remains of the original bilateral 

 symmetry of gills, auricles, and kidneys, there are genera which have come 

 down to us unchanged from Silurian times at least, e.g. Patella and Acmcea, 

 Pleurotomaria, Turbo and Trochus ; and scores of other Zygobranchiate 

 genera exist which only difier from their Cambrian ancestors in trifling 

 details of shell sculpture. Yet not one of these snails falls short of complete 

 torsion through 180°. 



It seems impossible to avoid the conclusion that the gap in the adult 

 succession between normal symmetrical MoUusca and Gastropoda is due 

 to some cause other than natural extinction or the imperfection of the 

 ' biological record ' to be read in the existing fauna. The gap marks an 

 evolutional saltation. The Gastropod accordingly is a ' sport,' and is 

 the consequence of a sudden jump in the evolution of the Veliger larva in 

 Cambrian, possibly earlier, times. With its ^asceral dome reversed this 

 new larva settled down and grew to maturity, the general course of growth 

 being unaffected by the change. When its growth fimished, the first 

 Gastropod had been created. How far the fiirst Gastropod differed in 

 other respects from its predecessor would require a long argument to tell, 

 except as regards one character to be dealt with in a moment. There 

 could be no trouble over its reproduction, since in Chiton and the Zygo- 

 branchs eggs and sperms are shed into the sea. The new characters were 

 presumably dominant : the recessives, if any are now left, are apparently 

 non-viable. Whether a gene was added, or dropped, I leave to geneticists. 

 At this stage I dare say the thought may be crossing the minds of some 

 of you that snails always have been queer-looking things, with something 

 abnormal about them, and that defiinitely to label them as ' sports ' will 

 not seriously disturb any cherished convictions. Even if the abnormality 



