ANATOMICAL PROBLEMS BEARING UPON EVOLUTION. 15 



Again, taking two variations in opposite directions 

 in a given organ, is the commoner variation of more 

 importance than the rarer ? Does the more frequent 

 illustrate a tendency in a particular direction on the part 

 of the species in which it occurs ? Are we justified in 

 concluding that of the two, that variation which occurs 

 in 65 per cent, indicates progressive change, and the 

 opposite, which occurs in 45 per cent., marks retrogressive 

 change in the species ? 



Teeth. — For example, it is much more common to 

 meet with individuals whose teeth are below the normal 

 number by reason of the rudimentary character of the 

 wisdom teeth, than those whose teeth exceed the normal 

 number. From this may we infer the existence of a 

 tendency towards diminution in the number of teeth? 

 Yes, we may, if the inference is strengthened by the com- 

 parison in size, form and number of the teeth of man and 

 other mammals. 



Hair. — It is similarly more common to find a diminu- 

 tion in the extent and quantity of hair than an excessive 

 growth on head, face or body. It may therefore be in- 

 ferred that man is gradually becoming less hairy, an 

 inference supported by comparative anatomy. 



Osseous Variations. — For the most part, however, 

 such inferences must be taken with the greatest caution, 

 and in regard to most of the variations which occur no 

 such inferences are possible. Xo evidence of the 

 existence of a special tendency exists. Osseous variations 

 appear to have a significance only within the narrowest 

 limits. For example, in the vertebral column two classes 

 of variations occur — correlated variations in the number of 

 vertebra? in each region, and unilateral variations of 

 individual vertebrae. The causes producing these varia- 

 tions are numerous, and may act separately or together. 



