Cicmings — Morphogenesis of Platystrophia. 127 



the P. dentata of the Niagara is almost absolutely invariable 

 in these respects.* 



A corollary of the law of progressive restriction of variabil- 

 ity is the rapid production of new types near the beginning 

 (during the epacmej of a phylum,.\ Thus the two primitive 

 species biforata and lynx were produced in the early Ordovi- 

 cian ; laticosta, costata, and dentata, in the Trenton-Lorraine ; 

 unicostata in the upper Lorraine, and acutilirata in the Rich- 

 mond. The entire Silurian famishes not a single additional 

 type. Bilobites, the only genus derived from Platystrophia, 

 originated some time during the Ordovician. The notion that 

 species are formed by very gradual increment of selected 

 variations certainly does not suit the present case. Many 

 of the earliest individuals of P. lynx and P. biforata are as 

 typical of those species as any that occur subsequently. So 

 many early specimens of P. laticosta and P. costata are typi- 

 cal representatives of their respective lines. The fundamental 

 difference between the early and later history of these types 

 is the presence of intermediate groups during the former 

 period and their absence during the latter. % 



It is possible that species may originate within a single genera- 

 tion. This is even insisted on by De Vries.§ It certainly has 

 occurred among domestic races, as Darwin || himself points out, 

 though systematists seem loth to admit that anything is a 

 species the origin of which is a matter of historical record ! 

 CopeT has suggested the sudden origin of genera through 

 the law of acceleration. Bailey,** in a very suggestive chapter 

 on experimental evolution, shows that many species of plants 

 originate suddenly under cultivation. I "do not wish to be 

 understood as maintaining that species habitually originate 

 after this fashion ; but cases such as that of Platystrophia 

 (and many similar ones will occur to the mind of every paleon- 



* Shaler's name, regiilaris, applied to the Anticosti form of this species, 

 probably alludes to this fact of invariability. 



f On this subject see Hyatt, Proc. Bos. Soc. Nat. Hist., xxxii, p. 371. — 

 Williams, Geological Biology, pp. 321, 322. — The fact has been noted by 

 many other paleontologists : Schuchert, Beecher, Hall, Clarke, Walcott, 

 Gratacap, etc. 



% Prof. H. S. Williams has shown that the Silurian-Devonian type Atrypa 

 reticularis affords a remarkable illustration of this law. (Geological Biology, 

 pp. 315-322.) The Devonian Spirifers, on the other hand, seem to be a group 

 in which segregation of species was more rapid, although S. mucronatus and. 

 S. disjunctus (Gosselet, Extrait des Mem. de la Soc. Geol. du Nord, iv, i, 

 1894) were certainly protean. A comparison of Hall's plates of Spirifer 

 (Pal. N. Y., vol. iv) with our fig. 27 shows that the latter presents material 

 enough for a dozen species, but for the connecting forms. 



§ Origin of species by mutation, Science, N. S., xv, No. 384, May, 1902. 



|i Animals and Plants under Domestication, 1868. 



^T Origin of the Fittest, 1886, p. 79. 



** Survival of the Unlike, 1896, pp. 107-137. 



