976 



PRINCIPLES OF STRATIGRAPHY 



resembling the adult of that genus which belonged in a lower hori- 

 zon. This occurrence threatened to cause difficulty in correlation. 



Parallelism and Convergence in Development. 



Homccogenesis. One of the most important of the many facts 

 brought to light by the researches of palaeontologists is that many 

 groups of organisms develop independently in similar directions, 



Fig. 259. Illustration of parallelism. A series of Eocenic Fusoid shells, be- 

 longing to different genera, but readily mistaken for members of 

 the same genus Fusus under which the\' were originally de- 

 scribed, a. Falsifusus meyeri; b. Fulgurofusus rugatus. c. 

 Fusus asper Y. ly^. d. Early whorls of same, enlarged X 10. 



and that hence in disconnected series species with similar adult 

 characteristics may develop. These morphological equivalents, as 

 Hyatt has termed them (or Homceomorphs of Buckman), are often 

 so much alike that they have been united under the same generic 

 or even specific name. The Eocenic fusoid shell Falsifusus, from 

 the Gulf Coast of the southern United States, is an example of 

 this, the adult form having all the characteristics of the genus 

 Fusus, though, as shown by the young stages, the^^American form 



