49 ANIMAL PHYSIOLOGY. 
not how, in obedience to the will of a First Cause. The other 
view is denominated the theory of descent with modification, 
the theory of transmutation, organic evolution etc., which 
teaches that all the various forms of life have been derived 
from one or a few primordial forms in harmony with the recog- 
nized principles of heredity and variability. The most widely 
known and most favorably received expvsition of this theory is 
that of Charles Darwin, so that his views will be first presented 
in the form of a hypothetical case. Assume that one of a group 
of living forms varies from its fellows in some particular, and 
mating with another that has similarly varied, leaves progeny 
inheriting this characteristic of the parents, that tends to be 
still further increased and rendered permanent by successive 
pairing with forms possessing this variation in form, color, or 
whatever it may be. We may suppose that the variations may 
be numerous, but are always small at the beginning. Since all 
animals and plants tend to multiply faster than the means of 
support, a competition for the means of subsistence arises, in 
which struggle the fittest, as judged by the circumstances, 
always is the most successful; and if one must perish outright, 
it is the less fit. If any variation arises that is unfavorable in’ 
this contest, it will render the possessor a weaker competitor: 
hence it follows that only useful variations are preserved. The 
struggle for existence is, however, not alone for food, but for 
anything which may be an advantage to its possessor. One 
form of the contest is that which results from the rivalry of 
members of the same sex for the possession of the females; and 
as the female chooses the strongest, most beautiful, most active, 
or the supreme in some respect, it follows that the best leave 
the greatest number of progeny. This has been termed sexual 
selection. 
In determining what forms shall survive, the presence of 
other plants or animals is quite as important as the abun- 
dance of food and the physical conditions, often more so. To 
illustrate this by an example: Certain kinds of clover are fer- 
tilized by the visits of the bumble-bee alone; the numbers of 
bees existing at any one place depends on the abundance of the 
field-mice which destroy the nests of these insects; the numbers 
of mice will depend on the abundance of creatures that prey on 
the mice, as hawks and owls; these, again, on the creatures that 
specially destroy them, as foxes, etc.; and so on, the chain of 
connections becoming more and more lengthy. 
If a certain proportion of forms varying similarly were sep- 
