412 SCIENCE PROGRESS 



probably a freely floating colony, since it terminates basally in 

 a sicula attached to a long thread ^ (nema). The repetition-series 

 of the numerous branches and the closely repeated hydrothecae of 

 the Dendrograptidi constituted the preliminary developments, 

 which by themselves were sufficient to induce a great amount 

 of variability. It is therefore not surprising that the true 

 Graptolites arose so suddenly and developed so large a variety 

 of forms, particularly since their ascendance into a numerous 

 and successful class was associated with the change from a 

 sessile, littoral habit (though perhaps attached to seaweeds) to 

 an exclusively pelagic, freely floating or even swimming mode 

 of life. This change was correlated with special adaptations 

 such as air-bladders (pneumatophores) and possibly even 

 swimming-membranes (Dichograptus). 



The earlier of the true Graptolites such as Bryograptus of 

 the Upper Cambrian (Tremadoc) among the Dicliograptidi or 

 Axonolipa^ are repeatedly branched, and as their evolution 

 proceeded we see the inevitable reduction and specialisation 

 in different directions rapidly taking place. The Dicliograptidi 

 stand nearest to the dendroid Graptolites and resemble them 

 in possessing no virgula and in the hydrothecae being in 

 contact back to back ; the extent of branching has already 

 become reduced in Tetragraptus and Loganograptus. Even 

 in this group a still further reduction has taken place from 

 diprionidian to monoprionidian forms ; in the later and still 

 more specialised order of the Axograptidi or Axonophora 

 (which, however, possesses a virgula giving greater rigidity to 

 the polypary) this reduction is still more pronounced. Some- 

 times it even becomes evident in a single individual colony, 

 as in Dicranograptus, in which the polypary is at first diprio- 

 nidian but soon divides distally into two monoprionidian 

 branches. 



It is in consonance with the principle discussed in this 

 paper that the progressive simplicity exemplified by the 

 reduced extent of branching, as well as by the reduction 

 of the series of hydrothecae from a double to a single row, 

 goes hand in hand with an increasing specialisation and com- 

 plexity of form of the hydrothecae. The earlier hydrothecae 

 were straight and possessed straight apertures ; as time pro- 



' This is true for D. JiabdUfoniic^ Eichw. ; in D. cavenwsum^ Wiman, 

 however, an adhesive disc is present. 



