CALIFORNIA ACADEMY OF SCIENCES. 245 



of species, on the mainland of California, were much de- 

 creased both by fires and herds of cattle, which were never 

 numerous on the four southern islands. The largest islands, 

 nearest land were, and still are, well stocked with cattle, and 

 shells are scarce on them. To the cattle must also be attrib- 

 uted the rarity of fossil specimens near the surface of the 

 mainland, while they abound on some of the islands. Such 

 as are found deeper, or in sheltered spots, are like the liv- 

 ing, but somewhat different living forms characterize 

 the mountains and the valleys. Some colonies in the low- 

 lands of Ventura and more northern counties, have escaped 

 destruction apparently by concealment under fallen trees in 

 the dry season, when the cattle sought wet pastures, and in 

 the wet season are little exposed to their trampling. Fur- 

 ther south they are found only where protected by rocks and 

 bushes on the sides of canons or mountains. Where stock- 

 raising has given place to agriculture, and fields are fenced, 

 they have become more numerous within thirty years. 



Looking at the whole subject of changes in the fossil and 

 recent land-shells, we find much to confirm Darwin's theo- 

 ries of the Origin of Species by the law of variation, nat- 

 ural selection, ami survival of the fittest. 



In an article on the "Distribution and Localities of West 

 Coast Helicoid land-shells,"* I stated that the interior valley 

 of California was destitute of them, on account of dryness. 

 This is true except of some very limited localities along the 

 central rivers and larger branches draining them, where 

 some of the species washed down from the mountains have 

 obtained a foothold on the edges of the great marshes which 

 compose so much of the low lands. It is apparent from the 

 rarity of these colonies, that they do not increase, but soon 

 die out, from want of some essential- element or food. No 

 species have, however, yet been found to have crossed from 

 one side of the valley to the other, or to have been washed 

 down far enough from the Sierra Nevada to have become 



*Amer. Jour, of Couch., IV, 211, 1869, and V, 207, 70. 



