THE PRODUCTION OF DOMESTIC RACES. 19 
tion is due to unbalanced natural selection or to some other principle, 
as, for example, the direct effects of use or disuse, or to indiscriminate 
elimination. Just as indiscriminate isolation may produce unbal- 
anced groups, and, therefore, segregation, so indiscriminate destruc- 
tion in the isolated groups is liable to produce unbalanced propaga- 
tion of diverse kinds, and so divergent transformation with intensive 
segregation. If animosities arise between two sections of a tribe, 
the domestic animals in the care of the two sections, though com- 
pletely isolated, may present no apparent differences till famine or 
some other calamity leads to the indiscriminate slaughter of all but a 
pair or twoof some species in one of those districts. This small 
fragment will, in many cases, be unable to reproduce in all respects 
the average character of the original race, and will become quite 
perceptibly divergent. If heavy but indiscriminate elimination falls 
upon the representatives of a given species in both sections of the 
country, the divergence in the isolated groups will be likely to be 
somewhat greater than if but one section suffers. This principle 
differs from natural selection in that the exclusion is indiscriminate 
instead of discriminate. Extreme elimination, leaving only a very 
small remnant, is always unbalanced elimination as regards some of 
the characters and, therefore, tends to produce transformation. 
Again, the effects of crossing between different strains and races of 
the same species may occur in different degrees in the different dis- 
tricts over which a species is distributed, and may, therefore, result 
in divergences in different districts. This principle I have called 
amalgamational transformation. 
4. The Stability of Races. 
For the preservation of a given race-type it is usually considered 
necessary to exclude from propagation a certain proportion of the 
variations that fall below the average which constitutes the type. The 
degree of exclusive breeding that is needed to maintain the present 
average depends upon the stability of the type, that is, the weakness 
of the tendency to revert to forms possessed by more or less remote 
ancestors. Whether a type may become so fully established as to 
maintain a constant average without any tendency to reversion, is 
perhaps an open question; but in the case of the goose, which is one 
of the most stable of domestic species, it is not certain that what has 
been called the birth average more closely resembles ancestral types 
than does the average that comes to maturity and propagates the 
species. The vitality of those that live and propagate is undoubtedly 
higher than that of the whole generation; but this does not prove 
