THE ORIGIN OF THE HUMAN BODY 297 



candidates for the position of ancestors, but, as no trace of 

 a third eyelid could possibly be left behind in the imperfect 

 record of the fossiliferous rocks (soft parts like this having but 

 slight chance of preservation) , we do not really know whether 

 the palaeozoic reptiles possessed this particular feature, or not. 

 Nor can we argue from analogy and induction, because not all 

 modern reptiles are equipped with third eyelids. Hence the 

 particular group of palaeozoic reptiles, which are supposed to 

 have been our progenitors, may not have possessed any third 

 eyelid to bequeath to us in the reduced and rudimentary form 

 of the plica semilunaris. If it be replied, that they must have 

 had this feature, because otherwise we would have no ances- 

 tor from whom we could inherit our semilunar fold, it is obvi- 

 ous that such argumentation assumes the very point which it 

 ought to prove, namely: the actuality of evolution. Rudi- 

 ments are supposed to be a proof for evolution, and not, vice 

 versa, evolution a proof for rudiments. 



Finally, the basic assumption of Darwin that the semilunar 

 fold is destitute of function is incorrect; for this crescent- 

 shaped fold situated in the inner or nasal comer of the eye 

 of man and other mammals serves to regulate the flow of the 

 lubricating lacrimal fluid (which we call tears). True this 

 function is secondary compared with the more important func- 

 tion discharged by the nictitating membrane in birds. In the 

 latter, the third eyelid is a pearly-white (sometimes trans- 

 parent) membrane placed internal to the real eyeUds, on the 

 inner side of the eye, over whose surface it can be drawn like 

 a curtain to shield the organ from excessive light, or irritat- 

 ing dust; nevertheless, the regulation of the flow of lacrimal 

 humor is a real function, and it is therefore entirely false to 

 speak of the semilunar fold as a functionless rudiment. 



The coccyx is likewise cited by Darwin as an example of 

 an inherited rudiment in man. "In man," he says, "the os 

 coccyx, together with certain other vertebrae hereafter to be 

 described, though functionless as a tail, plainly represents this 

 part in other vertebrate animals." (Op. dt., ch. I, p. 42.) 



