224 
BIOLOGY: A. H. CLARK 
Proc. N. a. S. 
Some of these crystallization centers or foci lie close together, as the 
crustaceans and the insects, while others are quite isolated, like the nemer- 
teans, phoronids, brachiopods, chsetognaths and echinoderms. They are 
not now connected by any intermediate forms, and there is no evidence 
that they ever were. Each represents a stable and economically efficient, 
circumscribed and distinctive, combination of structural characters crys- 
tallized out of a wide range of possibilities resulting from the disintegration 
of the coelenterate type which has, so to speak, gone into solution. 
Evolution, in the sense of progressive specialization, has been conclu- 
sively demonstrated within many groups, but always- within restricted 
sections of these groups. There is no evidence that the larger groups, or 
the broader divisions within these groups, succeeded each other in any 
such way. The appearance of all the major multicellular animal types 
was probably simultaneous or nearly so, the original progenitor giving rise 
continually to innumerable variations in form, symmetry and organization 
most of which proved economicalty impossible, but many of which, proving 
suitable to meet a certain range of conditions, formed centers about which 
the various phyla were developed. 
Whereas the animals between the coelenterates and the enteropneusts 
are commonly considered as representing progressive evolution, this is not 
so in fact. It is true that they represent increasing bodily efficiency, 
but this bodily efficiency results from progressive structural deficiencies 
which throw into relief the features upon which this bodily efficiency is 
based. All the essential features of the bilateral animals preexist in some 
form or other in the coelenterates, but they can only become effective 
economically through the partial or more or less complete inhibition of 
others which here hold them in check. 
Unimportant as they seem to most morphologists the possession of gill 
apertures characterizes a most interesting group of diverse animal types,, 
the pterobranchiates, balanoglossids, tunicates, cephalochordates and ver- 
tebrates. In all of these except the balanoglossids there is a sudden 
return to the fundamental colonial habit in a modified form, as if the 
appearance of gill apertures marked the beginning of the senescence of 
the animal line as a whole which from that point on recapitulated, or 
rather paralleled, its earlier stages. In the cephalochordates and verte- 
brates the internal colony or coelome becomes enormously subdivided, 
recalling more or less closely the conditions in the annelids; on the other 
hand in the tunicates there is practically no coelome, but the formation 
of colonies by budding, which are often highly complicated and with divi- 
sion of labor, reappears. In the pterobranchiates {Rhabdo pleura and 
CepkalodiscMs) the formation of colonies by asexual budding is developed 
to a high degree. 
From the preceding discussion it would seem that the only way by 
