Lhe Theory of Evolution 87 
interbreed, so that their descendants come to have, after a 
time, the common blood, so to speak, of all the new forms. 
If with each union there is a blending of the substances of 
the individuals, there will result in the end a common sub- 
stance representing the commingled racial germ-plasm. 
A new starting-point is then reached, and new species 
may continue to be formed out of this homogeneous ma- 
terial. Thus, in a sense, we have reached a position 
which, although it appears at first quite different from 
the ordinary view, yet, after all, gives us the same stand- 
point as that assumed by the transmutation theory ; for, while 
the latter assumes that the resemblances of the members 
of a group are due to descent from the same original 
form, and often by implication from a single individual, 
we have here reached the conclusion that it is only a 
common, commingled germ-plasm that is the common in- 
heritance. 
When we examine almost any group of living animals or 
plants, whether they are low or high in organization, we 
find that it is composed of a great many different species, 
and so far as geology gives any answer, we find that this 
must have been true in the past also. Why, then, do we 
suppose that all the members of the higher groups have 
come from a single original species or variety? Why may 
not all, or many, of the similar species of the lower group 
have changed into the species of the higher group, — species 
for species? If this happened, the resemblance of the new 
species of the group could be accounted for on the suppo- 
sition that their ancestors were also like one another. The 
likeness would not be due, then, to a common descent, and 
it would be false to attempt to explain their likeness as due to 
a common inheritance. But before going farther, it may be 
well to inquire to what the resemblances of the individuals of 
the original species were due; for, if they have come from an 
older group that has given rise to divergent lines of descent, 
