RODENTS IN SOIL FORMATION — GRIN NELL 347 



around the margin of the object, which thus, as the years go by, 

 gradually disappears, as a result of the process of undermining and 

 settling plus that of the building up of the ground roundabout. The 

 seeker need not go far to find good cases of this kind. Mr. Joseph 

 Dixon cites the case of a rock pile on one corner of his ranch which 

 was half buried in 10 years. One certain rock settled 6 inches in 

 comparison with the general land level during a period of 10 years. 



Some idea of the extent of the work which gophers have ac- 

 complished through time may be gained from the following consid- 

 erations. There are 33 species and subspecies of this type of bur- 

 rowing rodent now in existence in California; these are all in a 

 general way similar to one another, but each has distinctive charac- 

 ters which involve not only external features of color and quality 

 of pelage but also internal structures, more especially those of the 

 skull and teeth. These latter features, it will be observed, have to 

 do with the digging equipment which to the gopher is of such vital 

 importance. These characters of skull and teeth are the ones chiefly 

 depended upon by systematic students in determining species. A 

 remarkable thing is that no two of the 33 species occurring in Cali- 

 fornia exist in any one locality. Just one kind lives in a place, to the 

 exclusion of all other kinds. There is probably close correlation of 

 structure with peculiarities of the terrain, as, for instance, those 

 shown by loamy, sandy, and rocky soils. 



Now the remains of gophers' skulls are found in abundance in the 

 Rancho La Brea deposits near Los Angeles. The exact horizon in 

 which they are found is that in which are also found the saber-toothed 

 tiger, the ground sloth, camels, mastodons, and dire wolves. That 

 horizon has been assigned to the Pleistocene epoch; and geologists 

 have estimated that the time elapsed since the deposit of the materials 

 representative of that horizon is to be computed in " tens and even 

 hundreds of thousands of years." A remarkable thing is that a study 

 I have just made of the gophers of Rancho La Brea, in comparison 

 with the species existing in the same vicinity to-day, shows they are 

 identical in every respect. 3 In, say, 200,000 years there has been no 

 modification of the same structures which at the present day vary 

 from place to place to such an extent as to have led this group to 

 be characterized by some zoologists as extraordinarily " plastic." 

 The true inference here is that the processes of divergent evolution, 

 as well as of monotypic evolution, have been exceedingly slow. This, 

 however, is somewhat beside the issue. 



If we are to grant that gophers have been in existence, carrying on 

 their digging operations for so short a time, geologically, as 200,000 



3 The species and subspecies is Thomomya bottw paUesccns. 



