134 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, NO. 7 
All four of these intermediate types actually occur. All are entirely 
and widely distinct from each other, and show no demonstrable inter- 
gradation with any of the remaining animal types, each occupying 
a markedly isolated position. Three of the four are extraordinarily 
rich in genera and species; the last and least successful is represented 
by two closely parallel and non-intergrading forms which are identical 
with regard to the features with which we are concerned, but differ 
in all others. One of the first three is the only major animal group 
which has not persisted to the present time. 
The segmented animals with a coelome are the annelids; the un- 
segmented animals with a coelome are the priapulids and sipunculids; 
31 
let readjustment 
me 
~ 
~ 
ss 
~ 
eae, 
Fig. 1. Showing the first and second readjustments. 
the solitary acoelomate animals with abundant asexual reproduction 
are the rotifers; and the acoelomate animals forming colonies of sepa- 
rate individuals are undoubtedly the graptolites (fig. 3). 
Through this readjustment as just described each of the new animal 
types would combine the features of two of the original types. But 
since in each of these new types two of the four chief features are 
absent, there would still exist a condition of unstable equilibrium as 
compared with the colonial coelenterate-like ancestor. 
A second readjustment of the same nature as the first would be 
inevitable by which four animal types would appear in line with the 
first, but combining the characters of the intermediates in the second 
series (fig. 1). 
