GREAT STEPS IN ORGANIC EVOLUTION 825 



mutant types, as Morgan has so well shown for the Fruit-fly (Droso- 

 phila) and De Vries for the Evening Primrose (CEnothera), there are 

 some types which have persisted in much the same state of being for 

 inconceivably long ages. Thus the lamp-shell (Lingula), which still 

 flourishes to-day, projecting in multitudes from the mud of shallow 

 waters in the Indian Ocean, was established as a genus (if not even 

 species?) hundreds of millions of years ago in the Silurian Period. 

 There are other long-lived Brachiopods, such as Discina, Crania, 

 Rhynchonella (all three from the Ordovician onwards), and Tere- 

 bratula (from the Devonian). 



The Pearly Nautilus is the only Hving Cephalopod that shelters 

 itself inside a chambered shell, for the unchambered shell of the 

 Paper Nautilus (Argonauta) is formed by the female only, and is 

 more of a cradle for the young than a house for the adult. Now the 

 Pearly Nautilus, represented to-day by four species off Sunda and 

 Fiji Islands, dates from the Jurassic, and has near relatives much 

 more ancient. 



The Queensland Mudfish (Ceratodus) which has its swim-bladder 

 transformed into a lung, was widespread in the Triassic; and about 

 the same time there arose the rare lizard-Uke Sphenodon, still 

 holding its own (under protection) in two or three small islands in 

 the Bay of Plenty off the coast of New Zealand. It is the sole survivor 

 of its order of Rhynchocephalia, and so an extreme instance of what 

 Darwin caUed "living fossils". 



These are a few outstanding instances of persistent types, but 

 their frequent citation by palaeontologists is apt to give rise to the 

 misconception that long persistence is rare. As a matter of fact 

 there are numerous cases of long persistence. Thus there are some 

 very ancient types among Foraminifers and Radiolarians ; among 

 worms, e.g. Serpula, so common on shells; among minute crusta- 

 ceans, e.g. Cypridina of salt water and Pontocypris of fresh; among 

 bivalves, e.g. the hammer-shell Avicula; among Gasteropods, e.g. 

 the not infrequent Chitons, and, most familiar of all, the common 

 limpets of our rocky shores, whose type, or even genus, has continued 

 since Silurian times. There are also doubtless numerous persistent 

 types which cannot yet be demonstrated as such, owing to our still so 

 limited exploration of the geological record, itself so imperfect at best. 



How are we to explain the persistence of the same type, and 

 without appreciable change, for it may be millions of years ? These 

 types have proved themselves of normally sound constitutions, 

 effectively adjusted in adaptation to some fairly persistent environ- 

 ment, and hence cumulatively stabilised in their heredity. Given 

 such organisation in harmony with its long-enduring conditions of 

 life, what reason can there be for change? A life-equilibrium has 

 been reached, and the organism holds fast accordingly; its tendencies 

 to variabihty have fallen into abeyance. 



