CALIFORNIA ACADEMY OF SCIENCES. 23T 



show the effect of rivers in past times in transporting and 

 preserving them. 



It is noteworthy, also, that among all these, no extinct 

 species has yet been reported from the post-pliocene beds, 

 and those found living do not seem to have undergone any 

 changes to distinguish them from fossil forms. The links 

 connecting them with the extinct forms of the Tertiary 

 Age, are being discovered only on the west slope. 



Of the changes in species effected by the new conditions 

 caused by cultivation, there is as yet nothing recorded. As 

 a very large proportion of the forest species of the Atlantic 

 border are also found far out in the prairies of the Missis- 

 sippi Valley, with little or no shelter by trees, it is probable 

 that the majority of them can become accustomed to cleared 

 fields so as to survive about their borders, even without 

 showing any changes that might be attributed to an accom- 

 modating natural selection. Yet it is very probable that 

 such variations will occur, and must be sharply looked for. 



There is reason to believe that such changes in form, size, 

 and color have in recent times occurred in Europe from 

 such influences, and are even now producing many of the 

 so-called new species, sub-species, etc., constantly being 

 found, even in the older and best known regions. Varieties 

 have also attracted attention in the Eastern States, but have 

 been little studied, and are all supposed to be produced by 

 natural, not artificial influences, nor have they been regard- 

 ed as distinct species. 



A highly interesting and almost endless field for investi- 

 gation and experiment in this direction is open before us. 

 A branch of the subject relates also to the effects of our cli- 

 mate, etc., on introduced species, some of which have been 

 long acclimated without changing. The negative influence 

 of cultivation is also shown by the statement of Woodward's 

 Manual, that only three living European species have been 

 found to be extinct in England, and it is not supposed that 

 they were exterminated by cultivation. On the other hand, 



