538 Causes and Course of Organic Evolution 



In summary at this stage it may be said that four great 

 metazoan stocks or phyla have been recognized by the writer, 

 from some one of which the higher groups have evolved. These 

 are the Porifera, the Coelenterata, the Echinodermata, and 

 the Rotifera. Of these the first-named three failed to give 

 rise to polyphyletic and dominant descendants higher than 

 the most evolved members of their own division. The fourth, 

 by formation of specialized and concentrated sense-centers, 

 of active neuro-muscular tissue that was intimately related 

 to these centers, of complete alimentary canal with associated 

 glands that ensured high and continuous metabolic activity, 

 of ramified excretory organs that prevented accumulation 

 of waste products, and of specialized reproductive cells that 

 ensured dissemination and perpetuation of the species, became 

 a highly plastic, adaptable, and responsive group. 



In virtue of such high and varied environal relations, corre- 

 spondingly high and varied proenvironal responses were made. 

 So originated the diverse structural details and adaptations 

 that together constituted the Rotifera a polyphyletic ancestral 

 group, from which the Turbellaria (and through them the 

 Gastrotricha and Nemertinea), the Annelida, the Polyzoa, the 

 Brachiopoda, the Mollusca, and the Arthropoda have all sprung. 



But of these the two groups that most perfectly retained 

 the qualities of rapid and plastic environal perception and 

 proenvironal response were the Turbellaria and the Annelida. 

 For, while the Arthropoda retained this to a large degree, the 

 tendency to secretion of a chitinous or calcareous exoskeleton 

 has unquestionably proved a serious obstacle to plastic adapta- 

 tion, even though often aiding powerfully as a means of defense. 

 So it is to one of the above two groups that we should look 

 for an active and steady line of evolutionary ascent, that would 

 lead us to the higher or vertebrate phylum. But a parting 

 of the way occurred as they evolved from a common or related 

 rotiferan ancestry. For in simple Turbellaria the bdelloid 

 rotiferan dorso-lateral and ventral nerve cords were retained; 

 in simple Annelids, on the other hand, the ventral system 

 became the dominant and in time the only important longi- 



