600 



SCIENCE 



[N. S. Vol. XXXI. No. 798 



THE NEW YORK ACADEMY OP SCIENCES 

 SECTION OF BIOLOGY 



At the regular meeting held at the American 

 Museum on February 14, 1910, Professor C. C. 

 Curtis presiding, the following papers were read: 



Variability of Land Snails (Cerion) in the Ba- 

 hcrnia Islands icith its Bearing on the Theory 

 of Geographical Form Chains: Chaeles B. 

 Davenpoet. 



Professor Plate has described, in the Arohiv f. 

 Eassen- und GesellsohaftsMol., Bd. IV., the dif- 

 ferent forms of a genus of land snails (Cerion) 

 from the Bahama Islands; and declares that the 

 Gerions of the north coast of New Providence 

 constitute the best known and most known and 

 most manifold example of such a morphologic- 

 geographic " form chain " as the Sarasins describe. 

 Going from the west to the east end of the island 

 " regular and definitely directed changes " are 

 said to occur " conditioned by the amount of 

 precipitation together with an inner factor — a 

 high responsiveness of the protoplasm." 



In January, 1910, I collected shells in New 

 Providence from the localities specified by Plate 

 and from several others. I am now attempting to 

 breed them. Meanwhile the evidence seems op- 

 posed to Plate's view, since the " western " type 

 is found at various localities in the east alongside 

 of the eastern type. The facts seem to accord 

 better with the view of the immigration into the 

 eastern end of New Providence of snails having 

 the characteristics of Cerions from the Eleuthera 

 Island (an immigration facilitated by geographic 

 conditions) and by the formation of varied com- 

 binations of characters and pseudo-blends by 

 hybridization. 



Application of the Quadrate-incus Theory to the 

 Conditions in Theridont Reptiles and the Gen- 

 etic Relations of the Latter to the Mammalia: 

 W. K. Geeqoey. 



Eeichert's conclusion that the incus and malleus 

 of mammals represent the vestigial and meta- 

 morphosed jaw elements of lower vertebrates, to- 

 gether with the opposing view that these ossicles 

 in the mammalia have been derived directly from 

 the supra- and extra-stapedial cartilages of rep- 

 tiles, were considered. Exception was taken to 

 Dr. Broom's form of the latter theory, which took 

 the auditory ossicles of the crocodile as a theoret- 

 ical starting point. All the bones surrounding 

 these elements in the crocodile had evidently 

 undergone certain peculiar specializations and it 



would be surprising if the auditory ossicles them- 

 selves had not also sufi'ered considerable modi- 

 fication in the endeavor to evolve an improved 

 auditory apparatus; the resemblances in the 

 ossicles between crocodile and mammal may there- 

 fore be due chiefly to convergent evolution. The 

 modern upholders of the incus-quadrate, malleus- 

 articular theory demand for the ancestral mammal 

 a freely movable quadrate, similar to that of the 

 lizard; but this was because they seem to push too 

 far the biogenetic law. The incus or supposed 

 homologue of the quadrate at present appears in 

 the embryo as a freely movable bone, but this 

 does not prove that it has always been freely 

 movable. These investigators had passed by the 

 theridonts of the Permian and Triassic because in 

 these reptiles the quadrate was fixed at its upper 

 end; but a slight atrophy of the posterior border 

 of the squamosal would have greatly increased 

 the mobility of the quadrate. 



Paleontological and embryological evidence 

 showed that the existing joint between the skull 

 and the lower jaws in mammals is a neomorph, 

 probably developed pari passu with the atrophy of 

 the quadrate and articular bones. The application 

 of Eeichert's theory to the Theriodontia required 

 only that the vestigial quadrate should be freed 

 from its squamosal socket, and secondly that it 

 and the articular should be brought into contact 

 with the stapes or primary auditory rod. But 

 how can we conceive an adaptive, mechanical 

 motive for this extraordinary change? Such 

 seems to be furnished by the embryology of the 

 tympanic chamber of mammals. As is well knovsm, 

 this chamber appears below the ossicles as a 

 diverticulum of the first gill opening. It grows 

 upward and embraces the ossicles, which finally 

 appear to be inside the cavity but are morpholog- 

 ically outside of it, since they never pierce its 

 epithelium. So in the hypothetical pro-mammal 

 the vestigial quadrate and articular on the one 

 hand and the stapedial rod on the other may have 

 been embraced by the up-growing tympanic sack 

 or chamber and finally pressed into contact with 

 each other. The vestigial jaw elements may thus 

 have come to share in the vibrations of the cham- 

 ber and of the stapes, and thus was initiated their 

 career as accessory auditory ossicles. A somewhat 

 analogous case is the transformation in siluroid 

 fishes of certain vertebral appendages into a chain 

 of ossicles for transmitting vibrations from the 

 air bladder to the internal ear. 



L. HUSSAKOF 



