PERSISTENT TYPES OF LIFE 389 



the rapidity of that change will be measured by the demons- 

 trable amount of modification. On the other hand, it 

 must be recollected that the absence of any modification, 

 while it may leave the doctrine of the existence of a law of 

 change without positive support, cannot possibly disprove 

 all forms of that doctrine, though it may afford a sufficient 

 refutation of many of them. 



The PROTOZOA. The Protozoa are represented throughout 

 the whole range of geological series, from the Lower Silurian 

 formation to the present day. The most ancient forms 

 recently made known by Ehrenberg are exceedingly like 

 those which now exist : no one has ever pretended that 

 the difference between any ancient and any modern 

 Foraminifera is of more than generic value, nor are the 

 oldest Foraminifera either simpler, more embryonic, or 

 less differentiated, than the existing forms. 



The CCELENTERATA. The Tabulate Corals have existed 

 from the Silurian epoch to the present day, but I am not 

 aware that the ancient Helioliles possesses a single mark 

 of a more embryonic or less differentiated character, or 

 less high organization, than the existing Heliopora. As 

 for the Aporose Corals, in what respect is the Silurian 

 Paleocyclus less highly organized or more embryonic 

 than the modern Fungia, or the Liassic Aporosa than the 

 existing members of the same families ? 



The Mollusca. In what sense is the living Waldheimia 

 less embryonic, or more specialized, than the paleozoic 

 Spirifer ; or the existing Rhynchonellse, Granite, Discinse, 

 Lingul%, than the Silurian species of the same genera ? 

 In what sense can Loligo or Spirula be said to be more 

 specialized, or less embryonic, than Belemnites ; or the 

 modern species of Lamellibranch and Gasteropod genera, 

 than the Silurian species of the same genera ? 



The ANNULOSA. The Carboniferous Insecta and Arach- 

 nida are neither less specialized, nor more embryonic, 

 than these that now live, nor are the Liassic Cirripedia 

 and Macrura ; while several of the Brachyura, which appear 

 in the Chalk, belong to existing genera ; and none exhibit 

 either an intermediate, or an embryonic, character. 



The VERTEBRATA. Among fishes I have referred to the 

 Ccelacanthini (comprising the genera Ccelacanthus, Holo- 

 phagus, Undina, and Macropoma) as affording an example 

 of a persistent type ; and it is most remarkable to note the 

 smallness of the differences between any of these fishes 



