78 THE THEORY OF NATURAL SELECTION 
It is possible that variations of this nature may gradu- 
ally lead to important and even to specific changes, 
but whether this is the case still remains to be proved. 
On the other hand, we shall see that specific differences 
do sometimes arise at a single step, and there is strong 
but indirect evidence to show that this is the way in 
which a very great number of specific differences have 
actually arisen. Indeed, some have contended that 
this is the universal process by which such differences 
originate, but this again is not proved, nor is it 
altogether likely. In any case the inheritance of 
acquired characters can have nothing to do with that 
of definite and discontinuous differences. 
This is a problem to which we shall return in the 
concluding chapter, in the light of further evidence con- 
cerning continuous and discontinuous variations and 
their manner of inheritance, which will be by that 
time available. 
BIBLIOGRAPHY. 
Darwin, CHarLes: The Origin of Species, sixth edition, 1872. 
i * The Variation of Animals and Plants 
under Domestication, 1868. 
The Descent of Man, 1871. 
The Various Contrivances by which 
Orchids are Fertilized by Insects. 
Different Forms of Flowers. 
The Movements and Habits of Climbing 
Plants. 
oo 4 Insectivorous Plants. 
Life and Letters of Charles Darwin. 
ag More Letters of Charles Darwin. 
Mautuus, T. R.: An Essay on Population. 
Wa ttace, A. R.: Darwinism. 
