48 PROCEEDINGS OF THE ACADEMY OF [1879. 



variations, which the view here advocated, in some cases at least, 

 affords ; though I do not wish to be understood as saying that it 

 gives such an interpretation in a large proportion of instances 

 where the history of the interacting modifying forces are as yet 

 perhaps imperfectly known. Natural selection is quite adequate 

 to account for the development of an organ, or part, after it has 

 made its appearance, but it leaves the initial step causally unac- 

 counted for, which, it must be confessed, is the point where the 

 Lamarckian hypothesis seeks to supply the needed differentiating 

 causes. The hap-hazard, causeless variation of organisms cannot, 

 in the nature of things, exist; it is contrary to all known prece- 

 dent as exhibited in the phenomena of the inorganic world. 



In studying the teeth, one is confronted b}' a number of large 

 series of forms which clearty demonstrate the fact that large num- 

 bers of allied species which have succeeded each other in geologi- 

 cal time bear a genealogical relation to each other. The earliest 

 forms of teeth being the simplest, the later ones seem to have been 

 derived from them b}' a process easily understood, if mechanically 

 interpreted. The tooth earliest developed of all, seems to have been 

 a simple hollow cone superimposed upon a nutrient papilla; in- 

 deed the enamel and dentinal portion seems to be developed from 

 its superficial (epithelial) layers of cells which elongate as they 

 grow and crowd together, becoming columnar, whilst the excess- 

 ivelj- hard salts of lime constituting the dentine and enamel are 

 deposited around the columnar matrix of cells or odontoblasts by 

 secretion, leaving a fine tubular cavity in the centre, from which 

 the odontoblasts retreat as their substance is crowded out by the 

 formation of the hard material around them. In living teeth these 

 are always joined to the dentine fibrils, especially in those rootless 

 ones which are constantly growing and wearing away during the 

 whole life-time of the animal. It is true, as an objection to my 

 view, that the tooth is not protruded until its crown is in all re- 

 spects fully developed and hardened, but it is not improbable that 

 teeth, like bones, though apparently very hard, may be modified 

 by strains falling upon them persistently in approximately one 

 way for many hundreds of generations. The hardest substances 

 are elastic, compressible, and flexible, and I think it will hardly 

 be doubted that enamel and dentine in possessing these three 

 qualities, though manifested in an inconceivably small degree 

 under ordinary circumstances, may, when acted upon by forces 



