1 882.] On ArchcBsthetism. 457 



been discovered, and the homologies of their parts been ascer- 

 tained. 1 Perhaps the most important result of these investiga- 

 tions is the following : The variations from which natural selec- 

 tion has derived the persistent types of life, have not been general 

 or even very extensive. They have been in a limited number of 

 directions, 2 and the most of these have been towards the increase 

 in perfection of some machine. They bear the impress of the 

 presence of an adequate originating cause, directed to a special 

 end. Some of the lines struck out have been apparently inade- 

 quate to cope with their environment, and have been discontinued. 

 Others have been more successful and have remained, and at- 

 tained further modification. 



The reader can estimate the chance of the production of an 

 especially adaptive mechanism in the absence of any pressure of 

 force directing growth to that end. It appears to me that the 

 probability of such variation appearing under such circumstances 

 is very slight indeed, and its continuance through many geologic 

 ages directed to the perfecting of one and the same machine, still 

 smaller. For this reason, attempts have been made to demon- 

 strate a mechanical cause for the modifications of structure ob- 

 served. For these I refer to papers by Messrs. Alpheus Hyatt, 

 J. A. Ryder and myself; by Professor Hyatt * * "Upon the 

 effects of gravity on the forms of shells and animals;" 3 Mr. 

 Ryder " On the mechanical genesis of Tooth Forms ;" 4 and " On 

 the laws of digital reduction;" 5 by myself " On the origin of 

 the specialized teeth of the Carnivora;" 3 " On the origin of the 

 foot structures of the Ungulates ;" 7 " On the effect of Impacts 

 and Strains on the Feet of Mammalia." 8 Now demonstra- 

 tion of the mechanical effects of the application of force to mat- 

 ter can only be obtained by observation of the process, and this 

 cannot be seen, of course, by the observation of fossils. The 



1 Homologies and Origin of the Molar teeth of the Mammalia educabilia. Jour- 

 nal Academy Nat. Sciences Philadelphia, March, 1874. Proceedings Academy 

 ^•t. Sn..,S6 5 ,p. 22 . 



'See Hyatt on this point, Tertiary Planorbis of Steinheim. Anniv. Mem. Bost.. 



8 Proceeds. Amer. Assoc. Adv. Science, 1880, p. 527. 



