Miss Agnes Crane — Evolution of ihe Brachiopoda. 65 



this supposition is, that the distribution throughout interstellar space 

 is not permanent and does not follow the error law. If two bodies 

 flying about in space come into collision they may generate a 

 mass of gas, and if this mass of gas happens to get in the earth's 

 path it may be caught up. Indeed, if we assume that the earth's 

 atmosphere maintained a strict equilibrium with the interstellar 

 molecules at, say, a distance of 1000 miles from the earth's surface, 

 then a doubling of the almost infinitesimal pressure there, would 

 necessitate a doubling of the pressure at the earth's surface. Of 

 course this supposition is only used for the purpose of illustrating 

 the fact that a small alteration of interstellar pressure, if spi'ead 

 over a sufficiently vast space, might eventually give rise to a con- 

 siderable change in the atmosphere. Moreover, when considering 

 the applicability of Mr. Bryan's investigations, we must not forget 

 the enormous speed with which the earth meets the interstellar gases. 

 Hardly any molecules of those gases can be travelling at anything 

 like the speed with which the atmosphere charges into them. When 

 in addition I remember that the atmosphere is in a state of rotation, 

 during this charge into the interstellar matter, I comfort myself with 

 the thought that the problem of atmospheric interchanges is so 

 complicated that I may at all eA'^ents hope to enjoy my hypothesis 

 for a considerable time before anyone succeeds in giving it a really 

 decisive overthrow. Its great advantage as a theory of the Glacial 

 epoch is, that it does not require geographical changes such as are 

 usually postulated in connection with all other theories, even the 

 Astronomical one. 



III. — The Evolution of the Braohiopoda.* 



[A Sequel to Dr. Thomas Davidson's "What is a Brachiopod?"]* 



By Agnes Cranb. 



(PLATE IV.) 



Part I. 



FOR many years I have been deeply interested in the general 

 history of the Brachiopoda, and I have enjoyed the acquaint- 

 ance, correspondence, and friendly encouragement of most of the 

 eminent biologists and palasontologists who have devoted themselves 

 to the elucidation of the complex structure of this class. Here I 

 must name with reverence Davidson, long m}' friend and master ; 

 Barrande, of Prague ; James Hall, of Albany ; William King ; 

 Eugene Deslongchamps, the Norman naturalist ; Suess, of Vienna ; 

 Friele, of Bergen ; Morse, of Salem ; and Dall, of Washington ; 

 " all honourable men," and many others of the fin du siede school, 

 whose names will be noted in the sequel. 



During this period the history of the class has been completely 

 re-written. The Bi-achiopoda now seem to justify the prescience of 



' Eead at Chicago, August 24, 1893, before the Women's Auxiliary Branch of the 

 World's Congress (Section Geology), Department of Science and Philosophy ; also 

 before the Brighton Natural History and Philosophical Society, November 13, 1893. 



2 Geol. Mag. 1877, Dec. II. Vol. IV. pp. 145-155, and 199-208. 



DBCADB IT. — VOL. II. NO. II. 5 



