380 ORGANIC EVOLUTION CONSIDERED 



that the effects of wear have been inherited. To this 

 it has been objected by Tomes, " that it is quite im- 

 possible that the crowns of the teeth could have been 

 altered by impacts and strains, since their form is de- 

 termined in the recesses of the dental grooves, entirely 

 removed from all the mechanical influences which 

 affect the external surfaces of the jaws." * 



The existence of similar structures, which must 

 have had independent origins, has been regarded as 

 very strong evidence that they could not have been 

 evolved; yet the author enumerates f nine important 

 modifications of the skeleton, four of which have had 

 two distinct origins; one has had three; another, 

 four; two have had five; and one, six, and, probably, 

 eight, separate origins. Among these " The tongue- 

 and-groove ankle joint " " has been developed inde- 

 pendently along four distinct " lines. This is highly 

 improbable. 



The author concludes this matter by saying, " From 

 the preceding facts I have inferred that in biologic 

 evolution, as in ordinary mechanics, identical causes 

 produce identical results." J That the mechanical 

 causes in the above cases have been similar, is an 

 assumption. Besides, it is by no means certain that 

 mechanical causes could produce a tongue-and-groove 

 ankle joint once, and much less, four times. Accord- 

 ing to this theory, this kind of joint ought to have 

 been produced in other kinds of animals also, and in 

 the knees as well as the ankles. The principle, if 

 true, is of general application, and it ought to have 

 produced a greater number of similar results than we 

 actually find. His argument proves too much. 



In order to make his mechanical theory of varia- 

 tions available in evolution, it is necessary to prove 

 that slight changes thus produced are inherited by the 

 * Page 381. t Pages 360, 361. t Page 361. 



