APR. 4, 1923 CLARK: ORIGIN OF VERTEBRATES 133 
The four corners of this square are marked by four animal types 
which are closely related to each other, yet at the same time are funda- 
mentally distinct. One of them indicates the commencement of 
the segmented body; another shows the beginnings of the coelomic 
structures; a third is simple, with no indications of segmentation or of 
a coelome; while the fourth is a colonial form of the third. 
From these four points there would proceed evolution of two kinds. 
Each type would give rise to all economically possible variants 
through a process of continuous development which could be approx- 
imately represented by a branching tree-like figure; but all of the 
the different forms arising in this fashion would fall strictly within 
limits of its proper type. 
As examples of such evolution may be mentioned the insects, crus- 
taceans, molluscs, annelids, ete., within which groups all the included 
types may be more or less successfully represented as branches of a 
tree-like figure at the base of which lies a generalized or primitive 
form; but this and all the others however much they may diverge 
in details of structure always agree in their fundamental features. 
Since they all have arisen from the same colonial coelenterate-like 
ancestor which has, so to speak, exploded into four different types, 
each of these four points represents an animal complex in a state of 
unstable equilibrium; for each one has latent within it the fundamental 
features of the other three. 
Such an unstable equilibrium, in effect an explosive force, would 
presumably result in a sudden readjustment of the somatic balance 
whereby intermediate types would appear, one between each two of 
the four corners (fig. 1); while these intermediate types, each a dis- 
tinct re-creation and not genetically connected with either of its 
neighbors, would show a distinct economic advance, this economic 
advance would in no way represent real evolutionary progress, for it 
would be merely the result of the combination of the advantages 
inherent in the structure of the types on either side. 
Thus there would suddenly appear, quite without apparent ancestry, 
(1) segmented animals, like the tape-worms, with a coelome, like the 
flukes; (2) unsegmented animals, like the turbellarians, with a coelome 
like the flukes; (3) solitary unsegmented animals without a coelome, 
like the turbellarians, but with abundant asexual reproduction, like 
Microstomum: and (4) segmented animals without a coelome, like 
the tape-worms, but less unified and without the continuous loss of 
the units, as in Microstomum. | 
