INFLUENCE OF THE DESERT ON LIFE. 107 
It is not to be assumed, however, that external factors are without 
effect upon the evolution of living things. It has not yet been demon- 
strated that climatic or soil factors acting upon the somatic or vegetative 
part of the plant produce changes that are heritable, but on the other 
hand, MacDougal has shown that certain external agents, when brought 
to bear directly upon the germ plasm of the pollen tube or the embryo- 
sac of a plant, may produce profound alterations which are strictly 
inheritable after being tested to the third generation of the progeny. 
Solutions of zinc, calcium, and other substances combined were used and 
similar results followed the exposure to the action of radium. 
If we seek a like possible intervention of external forces which would 
act upon the plant unaided by man, we might find such influence coming 
from radio-active substances, such as spring and rain-water, or from the 
effects of sulphurous and other gases, which are being set free in number- 
less localities, or the protoplasts most nearly in contact with the egg 
apparatus may well excrete substances which would produce the same 
effect without regard to the forces which originally caused the distur- 
bances in the extra-ovular tissue. The actual technique of injection 
would be imitated in a measure by the action of foreign pollen which 
might find lodgment on the stigmatic surfaces and, sending down tubes 
through the style, introduce unusual substances into the vicinity of the 
egg-cell without participating in normal fertilization, which would ensue 
in the customary manner. Lastly it is to be said that it would appear 
that a most prolific source of such disturbances might be expected to 
result from the stings and lacerations of insects or the action of parasitic 
fungi, both sources of the most profound morphogenic alterations in 
somatic tissues, profusely exemplified by the well-known gall-formation 
of plants. (MacDougal, ‘‘Heredity and Origin of Species,” reprinted 
in advance from the Monist for January, 1906, and distributed Decem- 
ber 18, 1905; also ‘‘ Discontinuous Variation in Pedigree Cultures,’ Popular 
Science Monthly, September, 1906.) 
Tower (An Investigation in Evolution of Chrysomelid Beetles of the 
Genus Leptinotarsa, Carnegie Institution of Washington Publication 48, 
1906), after eleven years of assiduous experimentation with beetles, 
likewise concludes that influences merely exerted upon the body are 
without lasting effect in the history of a race and that they are not inher- 
itable. When such variations in moisture and temperature as would 
affect the germ-cells were brought to bear, however, profound changes 
ensued, which might be transmitted from generation to generation and 
which were not in the nature of adaptations. 
Formerly the desert was held to be an uninhabitable place, but by the 
aid of the devices of modern civilization the requirements of life, comfort, 
and luxury may be transported to the most remote deserts, and large 
populations may carry on pursuits, such as mining, unconnected with 
