342 



PRESENT PROBLEMS IN EVOLUTION AND HEREDITY. 



lateral toes, and concentration of tlie major axis of body weight and 

 strain npon the middle linger and toe. Man is also tending toward 

 monodactylism in the foot hy the establishment of the major axis 

 tlirongh tbe large toe and atrophy of the onter toes. The jiresent atro- 

 l>hy of our small toe is as good a parallel as we can find of the changes 

 which were occurring in the eocene period among the ancestors of the 

 horse. 



The Teeth. — But hoAv about the teeth, in which there is an absolute 

 loss of tissue in c<msequence of use? This is another objection raised 

 by Ball, Poulton, and others, which disap])('avs npon examination. 



The dental tissues, while the hardest in the body, and, unlike bone, 

 incapable of self-repair, are not only both living and sensitive, but, to 



A 



Kniliiis 



j ScnpliOirt llLinia) 

 Tilijj".' Troi>4 Mf^Cl"- Ciicif. 



Pollex /)irfc.r -Sled 



Fig. 



Primitive ungulate foot.— Lims of 

 vertical cleavage on either side of tin 

 middle toe, III. Spreading of toes would 

 cause separation of tbe (^arpala. 



Recent ungulate foot.— No inics 

 of vertical cleavage. All joints lirokcii 

 by ciilargeiiient of scaphoid, iincifonu, 

 and ladiu.s, tlie bones reseiving greatest 

 impact in walking. Lateral "toes, 1, ^■, 

 ilegciicrate. 



a very limited degree, plastic and capable of change of form. Ex 

 lu/pothcsi, it is not the growth, but the reaction tendency which pro- 

 (hu^es the growth, which is transmitted. The evolution of the teeth, 

 therefore, falls into the same category as bone.* In the accompanying 

 figures I have epitomized the slow transformation of the single-fanged 

 conical reptilian tooth, such as we see in the serpents, into the low- 

 crowned human grinder. We now know all the transition forms, so 

 that we can homologize each of the cusps of the human molar with its 

 varied ancestral forms in the line of descent. For example, the anterior 

 lingual or inner cusp of the upper true molars traces its pedigree back 

 to the reptilian cone. The anterior triangle of cusps, or trigon, seen in 

 the mesozoic mammalia and persisting in the first inferior true molar 

 of the modern dog, is still seen as the main i)ortion of the crown of the 

 human upjier molars {pr, pa, me). To this was added, ages ago, the 



* See especially tbe papers of Ryder, Cope, and the writer, "Evolntioii of Mam- 

 malian Molars to and from the Tritubercular Type," American Naturalist, 1889. 



