The Theory of Evolution 89 
. much boasted explanation of the resemblances of forms in 
the same group will be thrown into hopeless confusion. 
Let us ask another question in thisconnection. If a single 
species gave rise to a group of new species that represented 
the first vertebrates, they would have formed the first genus ; 
and if the descendants of these diverged again so that new 
genera were formed, then a group which we should call a 
family would have been formed. 
As the divergence went on, an order would be developed, 
and then a class, and then a phylum. The common charac- 
ters possessed by the members of this phylum would have 
been present in the original species that began to diverge. 
Hence, we find the definition of the phylum containing only 
those points that are the features possessed by all of the de- 
scendants, and in the same way we should try to construct 
the definition of each of the subordinate groups. This is the 
ideal of the principle of classification based on the theory of 
descent with divergence. If we admit the possibility of the 
other view that I have mentioned above, or of any other of 
the numerous possibilities that will readily suggest them- 
selves, then we must be prepared to give up some of the 
most attractive features of the explanation of resemblance 
as due to descent. 
That all biologists believe strictly in divergent descent, to 
the exclusion of any other processes, is not the case. And, 
as I have said before, since we are dealing with an historical 
question, it would be very unwise, in our present ignorance 
on many points, to pretend that we have any direct proof of 
the explanation that we find generally given to account for 
the resemblances of the species of a group to each other. 
At most we can claim that it is the simplest point of view, 
and that most biologists believe it to be also the most prob- 
able. It has been suggested that, in some cases, the new 
forms that arise from two or more species run a parallel 
course. If the original forms from which they came were 
