24 STUDIES IN EVOLUTION 



The simple antlers of the young Deer and Elk correspond 

 in type with those of the adults of the Middle Tertiary Deer 

 (Lydekker 44 ), and it may be therefore assumed that the great 

 number of branches and tines is a modern development. 

 Further back in the Tertiary the ancestors of the Deer were 

 without antlers, thus representing in phylogeny the new-born 

 Deer of the living type. These correlations are made from 

 comparisons of chronogenesis, or development in time, and 

 ontogenesis, or development in the individual. 



An example of a different kind will now be given to show 

 more clearly a genetic sequence in forms. Among the 

 Brachiopoda, Atrypa hystrix represents one of the terminal 

 members or species of a line of varietal and specific differ- 

 entiation, extending through the Silurian and Devonian. The 

 type commonly known as Atrypa reticularis appears to have 

 had its inception during the Ordovician ; yet in the Silurian 

 it is found as a conspicuous and fully developed form. Here, 

 also, it has quite a wide range of variation, but there seems 

 to be an insensible gradation between the extremes, which 

 therefore cannot be considered as definite permanent varie- 

 ties. There are, however, associated forms that have received 

 distinctive specific names, which do not shade into each other. 

 During the early and middle Devonian certain of these varia- 

 tions in the main stock of A. reticularis became more fixed, 

 and at the time of the Hamilton sediments in New York, 

 there are two forms known as A. reticularis and A. aspera, 

 which apparently do not pass into each other. As time went 

 on, these two types became more specialized and the diver- 

 gence correspondingly increased, until in the Upper Devonian, 

 in the Chemung sediments, there is a large many-plicated 

 A. reticularis, as well as a form with very few plications and 

 long marginal spines, A. hystrix. Hall and Clarke 31 thus 

 summarize the stages leading to the formation of the spinose 

 forms: "In the variant of Atrypa reticularis, occurring in 

 the Niagara fauna at Waldron, Indiana, the free concentric 

 lamellae frequently show a tendency to fold inward at the 

 summit of the principal plications. The infolded edges fail 



