LOCAL DISTRIBUTION OF SPECIES. 49 
dages. Asa destructive agency, the mammals of this region are respon- 
sible for losses which would be far greater were it not for the extraordinary 
degree of protection which so large a proportion of desert plants enjoy. 
The jack-rabbit, when pinched by hunger, attacks the flat opuntias of 
the hill and manages, in spite of thorns and spicules, to gnaw out large 
portions of joints, the plant presenting an unsightly appearance as the 
result, though never, as far as observed, entirely destroyed. 
The part played by rodents in the dissemination of seeds on the Labor- 
atory domain has probably been far greater than at first sight appears. 
Following the unusually heavy rains of the winter of 1904-05 the palo 
verde (Parkinsonia microphylla) on Tumamoc Hill bore an abundant 
crop of seeds, but on my return to Tucson in the fall it was with difficulty 
that a mere handful of pods could be found. The seeds are unprovided 
with any special means of dissemination, except as they contain food- 
substances, and there is no other explanation of their disappearance so 
probable as that they have been carried away for food by rats and squirrels, 
which are often very numerous in the vicinity of the Laboratory. In 
times of protracted drought, too, these animals are seen, emboldened by 
hunger, to carry away every vestige of possible food left on the ground. 
Following the dry winter of 1903-04 it was noticed that the sparse 
growth of annuals hardly appeared above ground before it was closely 
cropped; so that normal growth was quite impossible, owing to the 
presence of numerous half-starved rodents in search of food of any kind, 
which was eagerly snatched, even when they were watched at close range. 
At such times the destructive work of these animals becomes, it would 
seem, a factor of considerable magnitude in limiting the spread of certain 
species; but from their great abundance in following years of rainfall, 
it does not appear that the life of any species has been threatened. In 
short, the probability is that, with the exception of the giant cactus, 
which suffers, as already stated, from the work of wood-peckers, there 
are few, if any, species on the Laboratory domain that have been ser1- 
ously interfered with by animals. The case, of course, is different on 
the flood-plain of the river, where overpasturing has induced great and 
irreparable losses, but these changes belong to those induced by human 
agency and fall into a separate category. Allin all, the mutual relations 
of plants and the larger animals here are, at the present time, distinc- 
tively advantageous to each. 
As far as pertains to strictly local distribution, it is clear that ants, 
which are numerous both in species and individuals, are highly efficient 
agents. They are seen on all parts of the Laboratory domain, busily 
engaged in gathering seeds of various plants and carrying them to their 
quarters. Seeds of the following genera, thus carried by ants, have been 
identified: Microserts, Plantago, Lesquerella, Harpagonella, Cryptanthe, 
Daucus, Amsinckia, Festuca, Ervophyllum. 
