PHYLOGENY AND PALAEONTOLOGY 359 



hypothecated prorhipidoglossate mollusc, still less one 

 linking all the classes" (p. 79). 



Pointing in the same direction as the absence of transi- 

 tional forms is the undeniable fact that all the great 

 groups of animals appear with all their typical characters 

 at a very early geological epoch. Thus, in the Silurian 

 age a very rich fauna has already developed, and repre- 

 sentatives are found of all the main Invertebrate groups 

 sponges, corals, hydroid colonies, five types of Echinoderms, 

 Bryozoa, Brachiopods, Worms, many types of Mollusca 

 and Arthropoda. Of Vertebrates, at least two types of 

 fish are present Ganoids and Elasmobranchs. In the 

 very earliest fossiliferous rocks of all, the Precambrian 

 formation, there are remains of Molluscs, Trilobites and 

 Gigantostraca, similar to those which flourished in Cambrian 

 and Silurian times. 



The contributions of palaeontology to the solution of the 

 problems of descent posed by morphology are, however, not 

 all of this negative character. The law of recapitulation 

 is in some well-controlled cases triumphantly vindicated by 

 palaeontology. Thus Hyatt and others found that in 

 Ammonites the first formed coils of the shell often reproduce 

 the characters belonging to types known to be ancestral, and 

 what is more they have demonstrated the actual occurrence 

 of the phenomenon known as acceleration or tachygenesis, 

 often postulated by speculative morphologists. 1 This is the 

 tendency universally shown by embryos to reproduce the 

 characters of their ancestors at earlier and earlier stages in 

 their development. 



The most valuable contribution made by palaeontologists 

 to morphology and to the theory of evolution arose out of 

 the careful and methodical study of the actual succession of 

 fossil forms as exemplified in limited but richly repre- 

 sented groups. Classical examples were the researches of 

 Hilgendorf- on the evolution of Planorbis uiultifonnis in the 

 lacustrine deposits of Steinheim, those of Waagen 3 on the 



1 Particularly by E. Perrier, " La Tachygenese," Ann. Sci. nat. 

 (Zoo/.) (8), xvi., 1903. 



2 Monatsber. k. Akad. IViss.^ Berlin, pp. 474-504, 1866. 



3 Geognost. u. Palceont. Beitriige^ ii., Heft 2, pp. 181-256, 1869. 



2 A 



