PROCESSES OF EVOLUTION 



247 



added on to the end of this. Spath points out, however, that Hyatt used 

 this principle in classifying his ammonites into series of ancestors and 

 their descendants, and has therefore no right to use the phylogenetic 

 lines thus obtained to prove the correctness of the principle. 



Fig. 109. Semi-diagrammatic drawings of graptolites to illustrate some of the 

 "orthogenetic trends" in the evolution of the group. The time sequence runs 

 from A to J. A Pendent, many-branched form (Bryograptus); 6 pendent four- 

 branched form (Tetragraptus); C scandent four-branched form (Tetragraptus); 

 D and £ pendent and scandent two-branched forms (Didymograptus); f, G, and H, 

 transition from scandent two-branched to single-branched forms ; / and j, transition 

 to one-sided, one-branched forms (Dimorphograptus to Monograptus). Note the 

 two main trends, from pendent to scandent growth, and reduction in number of 

 branches; and note further that they are to some extent independent (compare 

 B and C; D, £, and F). 



Another famous example of programme evolution is that of the 

 Graptolites.^ These were colonial organisms, probably allied to the 

 Coelenterates, but perhaps more safely placed in a phylum of their 

 own. The colonies consisted of branches hanging from an attachment 

 organ which probably adhered to seaweed; along the branches the 

 individual polyps were arranged, each in a small cup or theca. There is 

 ^ Elles 1922, Bulman 1933. 



