212 AMERICAN JOURNAL 



He finds also that if he travels close to the ocean, new zones 

 appear to be constantly beginning, that is, new groups of species, 

 commencing at successive points along the coast, cross his path 

 obliquely, to extend inland for a greater or less distance, until they 

 disappear on reaching the point where the climate becomes unsuited 

 to their existence, or some other obstacle, such as water, unsuitable 

 soil, or insurmountable deserts interrupt their course. Thus the 

 traveller,whether he goes parallel to the coast line, or at right angles 

 to it, finds most of the species he encounters limited in their dis- 

 tribution, but by following obliquely the various zones corres- 

 ponding to the isothermal and isohyetal lines, may find the range 

 of species extending from fifty to a thousand miles. Sometimes 

 he passes over wide intervals in which a species seems to have 

 disappeared entirely, but on ascending another range of moun- 

 tains, and entering a region of similar climate, again discovers 

 the same species as numerous as in the region where he first en- 

 countered it. Or he may find in its place a closely analogous, 

 though constantly different form, which raises curious inquiries 

 in his mind as to the limits of specific differences, the influence 

 of external causes on their variations, and the question whether, 

 after all, they are not the creation of those combinations of influ- 

 ences exerted by the climate and soil together. Or he imagines 

 some means for their transportation across vast intervening 

 wastes, and supposes that, once transported, the new influences 

 around them have effected the changes he observes.* 



Among all the creatures he discovers,' perhaps none are more 

 interesting in these investigations as to the origin of species (so 

 important to the history of mankind himself), than the humble 

 and often despised snails. They have but slight powers of locomo- 

 tion, are short-lived and easily destroyed by any derangement of 

 the conditions favorable to their existence. It is easy to collect 

 and preserve their shells, and these may also be found fossil as 

 evidence of the geological period during which the species may 

 have existed, or of the changes it has undergone since creation.* 

 And though usually short-lived, many have wonderful tenacity 

 of life when in a torpid condition from cold or dryness, having 

 been known to revive after sleeping six years in cabinets without 

 food. (Stearns, Proc. Cal. Acad., 1867). 



* Mr. R. H. Stretch has recently brought from near Carson Valley, 

 Nevada, lat. 39°, fossils, or rather casts, closely resembling the Holospira 

 Newcornbiana and H. irregularis, Gabb, of Lower California. They 

 occur, he says, in the same formation that contains Carinifex and other 

 well known northern fresh water species, though none are in these frag- 

 ments. The investigation of these deposits, which are widely spread in 

 Nevada, will probably reveal a very different climate as existing there 

 within post-pliocene times. No Cylindrellidce now exist north of lat. 

 32° on the west side of the Rocky Mountains. 



