EVOLUTION OF THE COLOIJS OF BIRDS. 4/ 



occasions .■^•" He says: '* The evidence is of a direct and 

 indirect character. The direct evidence is that by actual 

 observation in complete paleontological series, the origin 

 of adaptive structures is found to conform strictly to the 

 lines of use and disuse. The indirect proof is that the 

 natural selection of chance variations is unsupported by 

 observation and is inadequate to explain the various 

 phenomena of the second class." Special attention is 

 drawn to the evolution of teeth, in which every grada- 

 tion may be traced from the simple conical reptilian 

 tooth to the highly complex molars of some mammals. 

 Osborn has enunciated the two following laws of cusp 

 growth: 



"(1) The primary cusps first appear as cuspules, or 

 minute cones, at the first points of contact between the 

 upper and lower molars in the vertical motions of the 

 jaw. 



(2) The modeling of cusps into new forms, and the 

 acquisition of secondary position, is a concomitant of 

 interference in the horizontal motions of the jaws." 



From the above laws it is evident that the variations 

 in the race are the same as the variations in the indi- 

 viduals, caused by the use and disuse of parts, and a 

 causal connection between the two is inferred. This 

 proof of the inheritance of the characters of use and dis- 

 use has been criticised by Poulton. In a foot-note to 

 Weismann's Essays upon Heredity,! he says: " One of 

 the most remarkable forms of this revival of Lamarck- 

 ism is the establishment in America of a ' Neo-Lamarck- 

 ian School,' which includes among its members many 

 of the most distinguished American biologists. One of 

 the arguments upon which the founders of the school 



■^ Proc. A. A. A. S., 1889, pp. 27:3-276. Am. Nat. March, 1891, p. 191. 

 t Page 4.S7. 



