88 Evolution and Adaptation 
then we are only removing the explanation one step farther 
back. If this original group has come from numerous species 
of a still older group, and this, in turn, from an older one 
still, then we must go back to the first forms of life that ap- 
peared on the globe, and suppose that the individuals of these 
primitive forms are the originals of the species that we find 
living to-day. For instance, it is thinkable that each species 
of vertebrate arose from a single group of the earliest forms 
of life that appeared on the surface of the earth. If this 
were the case, there must have been as many different kinds 
of species of the original group as there are species alive at 
the present time, and throughout all the past. This view finds 
no support from our knowledge of fossil remains, and, al- 
though it may be admitted that this knowledge is very in- 
complete, yet, if the process of evolution had taken place as 
sketched out above, we should expect, at least, to have found 
some traces of it amongst fossil forms. Since this question 
is an historical one, we can, at best, only expect to decide 
which of all the possible suggestions is the more probable. 
We conclude, then, that it is more probable that the verte- 
brates, the mollusks, the insects, the crustaceans, the annelids, 
the ccelenterates, and the sponges, etc., have come each from 
a single original species. Their resemblances are due to a 
common inheritance from a common ancestral species. Even 
if it be probable that at the time when the group of verte- 
brates arose from a single species, there were in existence 
other closely related species, yet we must suppose, if we 
adhere to our point of view, that these other related species 
have had nothing to do with the group of vertebrates, but that 
they have died out. Moreover, we must suppose that each 
order, each class of vertebrate, has come from a single origi- 
nal species; each family has had a similar origin, as well as 
each genus, but, of course, at different periods of time. Let 
us not shrink from carrying this principle to its most extreme 
point, for, unless the principle is absolutely true, then our 
