6 PHILOSOPHICAL SOCIETY OF WASHINGTON. 
to the Carditide, and living on the backs of Haliotis shells. The 
female has the base of the shell pushed up into a deme in the median 
line, the opening to which is closed by an extension of the mantle. 
In the pocket so formed the young of the species are protected by 
the mother. The only other case among the lamellibranchs of such 
a protective modification is that of Thecalia concamerata, in which 
the same end is reached in a different manner. Both belong to the 
same family. The males of Milneria are without the pocket. Both _ 
sexes adhere by a byssus. 
Mr, W. B. Tayxor read a communication on 
GEOLOGICAL AND PHYSICAL THEORIES, 
in which, controverting the claims of practical or field geology—to 
the exclusion of physical theory—in the solution of physiographic 
problems, he contended that the family ties of planetary relationship 
cannot be disowned by geology. He thought the value of “ exter- 
nal” inductions fully shown by the probable effects of varying 
eccentricity in the earth’s orbit on secular changes of climate, as 
well as by a reference to the general inter-relation between the 
meeting boundaries of astronomical, geological, physical, and chem- 
ical science. On the physical side, he maintained that the sup- 
posed demonstrations of the earth’s comparatively recent consol- 
idation, (as well as of the limit assigned to the sun’s active life,) 
were entirely inconclusive: first, from the admitted uncertainty of 
the data, and secondly, from our ignorance that unknown factors 
might not enter into the problem. He therefore heartily agreed 
with Captain Dutton in recognizing the strong demands of geological 
induction for an incomparably longer chronology than terrestrial 
physics could as yet cipher out.* At the same time, the speaker 
contended that the certainty remained entirely unimpaired of an 
origin and a limit to solar—as well as to planetary—energy ; unless 
we were prepared to accept the absurdity of an infinite potential. 
He also pointed out that the doctrine of “uniformitarianism” does 
not require (as sometimes too readily supposed) an unvyarying 
degree of energy in geological dynamics throughout the distant past; 
but that the contrary was the more probable—if only from the broad 
generalization that all action whatever has its period or periods of 
maximum and minimum. » 
* Mr. Taylor’s paper was a reply to one by Mr. Dutton, of which the 
Society obtained no abstract. See pp. 4 and 5. 
