128 Ou in nigs— Morphogenesis of Platystrophia. 



tologist) certainly point to the conclusion that given a new and 

 vigorous stock in a favorable environment, the initiation of 

 new species takes place with great rapidity. It does not mat- 

 ter whether the new stock has been derived by migration from 

 some outside region or as an offshoot of an endemic stock, so 

 long as its physiological peculiarities, or physical isolation, 

 remove it sufficiently from competition with other stocks.* 



After the initiation of new types, their segregation from each 

 other into definite groups (species) was relatively slow. Inter- 

 mediate forms persisted for some time. Segregation was not 

 accomplished until most of the characters of the types had 

 become fixed. 



Intrinsic evolution. — Certain phenomena of the evolution of 

 Platystrophia find their explanation in the progressive develop- 

 ment or accentuation of characters of high physiological im- 

 portance along the same line in different members of the 

 genus, especially during its epacme. These are the phenomena 

 of morphological equivalence^ and homo2omorphy,% of both of 

 which Platystrophia affords excellent examples. 



By morphological equivalence is meant the occurrence of 

 similar forms in similar succession under like conditions of 

 environment ; and by homceomorphy, similarity of species in 

 general features with dissimilarity in details. P. lynx and 

 P. biforata present examples of both. These two species are 

 perfectly distinct ; nevertheless, the fact that the early rep- 

 resentatives of P. lynx in this country have been persistently 

 referred to P. biforata, while the later Russian Ordovician 

 representatives of P. biforata have been as persistently referred 

 to P. lynx, is good evidence of the parallelism of the two \ 

 and the similarity of the sediments in which they occur and of 

 the faunas with which they are associated, is evidence of the 

 similarity of environmental conditions. Both species run 

 through a similar series of changes. Since they were derived 

 from the same radical they were of course alike at the outset. 

 In the middle Ordovician of Russia (see fig. 6 ante) we have 

 a form which in size, gibbosity, incurvature of the beaks, num- 

 ber of plications, shell index, and cardinal angle is the hornce- 



* Thus Plat ijstrophia was a highly anomalous type of Orthid, a fact which 

 gave it a distinct advantage. A factor that may have played an important 

 part in the rapid evolution of new types, since it furnishes precisely the 

 favorable conditions of physical environment demanded, has been pointed 

 out by Prof. T. C. Chamberlin (Jour. Geol., vi. 1898, pp. 600-G04). This is the 

 extension of submarine shelves and epicontinental seas during periods of 

 base-leveling and transgression. Such conditions existed, according to 

 Chamberlin, in the middle Ordovician, the very time when Platystrophia 

 attained its greatest expansion. See also Beecher, this Journal, vi, 1898, pp. 

 132, 133. 



f Hyatt, Genesis of the Arietidse, Mem. Mus. Comp. Zool., xvi, No. 3, 

 1889, p. viii. 



% Buckman, Homceomorphy among Jurassic Brachiopoda, Proc. Cottes- 

 wold Nat. Field Club, xiii, 4, 1901, pp. 231, et seq. 



