48 CALIFORNIA ACADEMY OF SCIENCES. 



have chiefly relied is derived from the comparative mor- 

 phology of mammalian teeth. The evolution of the 

 various types are believed to be due to modifications in 

 shape, produced by the action of mechanical forces 

 (pressure and friction) during the life of the individual. 

 The accumulation of such modifications by means of 

 heredity explains the forms of existing teeth. 



11 It may be reasonably objected that the most element- 

 ary facts concerning the development of teeth prove that 

 their shapes cannot be altered during the lifetime of the 

 individual, except by being worn away. The shape is 

 predetermined before the tooth has cut the gum. Hence 

 the Neo-Lamarckian School assumes, not the transmis- 

 sion of acquired characters, but the transmission of 

 characters which the parent is unable to acquire!" 



In replying to the criticism of Mr. Poulton, Osborn 

 says: "To the objection that the teeth are entirely 

 formed before piercing the gum, and that use produces 

 an actual loss of tissue as contrasted with the growth of 

 bone, it may be said that by our theory, it is not the 

 growth itself but the reactions which produce this growth 

 in the living tissue, which we suppose to be transmitted." 

 Osborn also criticises Weismann's theory as follows: 

 u lii Weismann's variation theory the preponderating 

 influence must be conservative; however it may explain 

 progressive modification, or even correlation of old char- 

 acters, it does not admit that the genesis of new char- 

 acters should follow definite lines of adaptation which 

 are not pre-existent in the germ-plasma. We find that 

 new characters of the second class do follow such pur- 

 posive or directive lines, arising simultaneously in all 

 parts of the organism, and first appearing in such min- 

 ute form that we have no reason to suppose that they 

 can be acted upon by selection. The old view of nature's 

 choice between two single characters, one adaptive, the 



