358 RECORDS. 



MALS IN THE RoCKY MOUNTAINS AND THEIR BeARINGS ON THE 



Present Problems of Evolution. 



F. B. Sumner, Experimental Studies of Adaptation and 

 Selective Elimination in Fishes. 



Summary of Papers. 



Professor Osborn exhibited the newly prepared skulls of Dip- 

 lodoacs, Morosaurus and Crcosaiiriis from the Bone Cabin Quarry, 

 Wyoming. The skull of Morosaunts is new to science, and is 

 of a short-skulled type with a very prominent and convex fore- 

 head. Like Diplodociis it exhibits a large pineal foramen. 



Under the title ** Recent Discoveries of Extinct Animals in 

 the Rocky Mountain Region and their Bearings on the Present 

 Problems of Evolution," Professor Osborn exhibited a series of 

 skulls of the Eocene ancestors of the Oligocene Titanotheres, 

 stating as a result of recent investigation that the Oligocene 

 Titanotheres were found to represent four distinct lines of 

 descent, in each of which horns independently developed, and 

 that the Eocene Titanotheres also represented four distinct lines 

 of descent, two of which became extinct, namely, the extremely 

 short-skulled PalcEosyops, and the extremely long-skulled Doli- 

 chorhimis, while the intermediate forms Telniatotlicriiun and Maii- 

 tcoccras gave rise to the Oligocene forms Titanotlieriinn and Mcga- 

 cerops respectively. As bearing upon the general problem of 

 evolution, it was pointed out that the palaeontologist enjoys the 

 peculiar advantage of following a series through the origin and 

 development of organs to their subseqent progression or decline. 

 As early as 1888 the speaker had taken the ground that various 

 palaeontological series demonstrate the defijiitc or determinate 

 variations of certain kinds. In 1892 he connected with 

 this the idea that certain series of animals related by descent 

 from a common stem form exhibit the potential of similar evoln- 

 tion, describing this as a law of latent or potential homology. 

 It is now found in this series of Titanotheres that there is more 

 than a potential of similar evolution ; there is evidence of a pre- 

 disposition to similar evolution as shown in the wholly independent 

 development in two distinct series of horns from hornless types 



