878 American Work on Recent Mollusca in 188 1. [November, 



ence (vol. xxix, Boston meeting) published June, 1881, is a com- 

 bination of an abstract of the lecture, together with " remarks 

 upon the effects of gravity upon the forms of shells and animals 

 (pp. 1-24, PL 1— 11, separate copies). In this extremely suggestive 

 paper, Professor Hyatt, after discussing the particular case of the 

 Steinheim Planorbis, strives to " bring into comprehensible shape 

 the following conceptions." The conceptions are chiefly to the 

 effect that the unsymmetrical spirality of most gasteropod shells 

 is due to the effects of gravity transmitted by heredity. That 

 many locally constant characteristics are due solely to the physi- 

 cal influences of the environment. That natural selection does 

 not explain these relations, but only serves to fix the results and 

 bring them within the reach of heredity, when they may be in- 

 herited according to the law of heredity with acceleration. That 

 gravity appears to be one of the causes of the differences in effort, 

 function and anatomy observed between various parts of animal 

 forms when laterally or vertically considered. That the bilateral 

 or geonialic (the tendency to equalize the form in direction of a 

 horizontal plane) growth of organs or organisms appear to be 

 directly or indirectly responses to the demands of gravity. Lastly, 

 that the origin of the limbs in pairs, while mere buds, perhaps, 

 may be the results of attempt at maintaining equipoise by geo- 

 malic growth in obedience to the laws of gravity. 



That this effect of gravity is marked in animals which become 

 permanently and immovably fixed, like the oyster (and only after 

 they become attached) Professor Hyatt shows to be the case in 

 many instances, and that he has suggested an hitherto overlooked 

 vera causa there seems to be no doubt ; though its effect in mod- 

 ifying, for instance, the cone of molluscan shells, seems less likely 

 than that the inevitable divergencies from a true cone produced 

 by the physical necessities of the environment, in perhaps a 

 majority of cases, were seized on by natural selection on account 

 of the advantages gained by economy in material, in space occu- 

 pied, strength resulting and protection insured to delicate inter- 

 nal organs by the spirality of the shell. Supposing all conchifers 

 to be born with a straight conical shell, it is self-evident that un- 

 less the creatures were pelagic or very sedentary, that fractures 

 and unequal developments of the margin of the cone would oe 

 the case in a majority of individuals. That in fact the conical 

 form would be a decided disadvantage to any creature which had 



