852 



SCIENCE. 



[N. S. Vol. II. No. 51. 



summit of a rock, looks the size of a yak or a 

 bear. ' ' 



It is plain from this experience that M. Bon- 

 valot happened upon a new spatial world of 

 size and distance, which he had to learn by a 

 method of local visual signs, just as in infancy 

 he learned the space world of the nursery room. 

 It would be interesting to inquire of such travel- 

 ers the exact nature of the signs they used in 

 constructing the new space world. 



HiEAM M. Stanley. 



ME. SPENCEE ON TACTUAL PERCEPTION AND 

 'natural SELECTION.' 



Mr. Spencer concludes his long discussion 

 on ' Natural Selection ' by a short note in the 

 October number of the Contemporary Review in 

 which he claims that he has received from 

 Prof Weismann no answer to the crucial ques- 

 tion he asked in his original paper {id, Feb. , 

 1893). Mr. Spencer writes: 



" But the main question he has every time passed 

 over in silence. To my repeated inquiry— -How are 

 the various degrees of tactual discriminativeness possessed 

 by different parts of the outer surface of the body to be 

 explained by ' natural selection ' or by 'panmixia ' ? he 

 has not only given no answer, but he has made no at- 

 tempt to give an answer. The obvious implication is 

 that no answer can be found. ' ' 



Now, as I have already attempted {Mind, 

 Oct., 1893,) to prove that Mr. Spencer's argu- 

 ments from tactual perception are futile, and as 

 his reply {Contemporary Review, Dec, 1893,) 

 shows that he is not likely to be influenced by 

 such evidence as I am able to adduce, I do not 

 return to the subject in the hope of convincing 

 him. I may, however, be able to show others 

 that the facts of tactual perception have no 

 special bearing on the sufficiency or insufficiency 

 of natural selection. 



Mr. Spencer found that the sensation areas 

 (the distance apart at which points on the skin 

 can be distinguished) on the tips of the fingers 

 of two blind boys were ^V "ic'^ ^iiid of two com- 

 positors yV inch, whereas Weber gave -}^ inch as 

 the normal size. Mr. Spencer concludes from 

 this experiment that the structure of the peri- 

 pheral nerves and their connections are altered 

 by use, and that these modifications of structure 

 are hereditary. The fact that the tip of the 



tongue is more sensitive than the tips of the 

 fingers is said to be because the tongue is con- 

 tinually exploring the teeth, although no ad- 

 vantage is gained thereby; the nose is more 

 sensitive than the top of the head because it is 

 more often rubbed by the fingers, etc. Mr. 

 Spencer says that as the sensitiveness of the tip 

 of the tongue is less important to man than sen- 

 sitiveness of the finger tips it is impossible that 

 the greater sensitiveness of the tongue could 

 have been developed by the survival of useful 

 variations. 



Now this argument is such that the only rea- 

 son for replying to it is that it is advanced by 

 Mr. Spencer, whose contributions to philosophy 

 are on the whole so important, that his utter- 

 ances on special matters carry weight that they 

 often do not intrinsically possess. 



The experiments and theories of Weber have 

 long since been superseded. Many thousands 

 of experiments on tactual discrimination by 

 a score of investigators have been published, 

 and of these Mr. Spencer is ignorant. It is 

 well known that the tactual discrimination of 

 the blind is likely to be greater than that of 

 others, but this could not have been determined 

 from an experiment such as Mr. Spencer made. 

 Tactual discrimination decreases in five min- 

 utes' practice far more than the amount given 

 by Mr. Spencer as the greater sensitiveness of 

 the blind ; but this does not mean that the an- 

 atomical structure of the peripheral nerves has 

 been modified, and that this modification will 

 be hereditary. 



The distribution of tactual discrimination on 

 the skin seems to be exactly what would be ex- 

 pected were ' natural selection ' a sufficient or an 

 insufficient account of organic evolution. The 

 parts of the body in which sensitiveness is most 

 useful, the finger-ends and the tongue, are in 

 fact the most sensitive. 



There are two adequate reasons why the 

 tongue should be more sensitive than the fingers. 

 In the first place the lower mammals use the 

 tongue as an organ of touch, it being far more 

 sensitive than their hoofs or paws; a horse will 

 reject the smallest bit of gravel from its mess of 

 oats. As sensitiveness of the tongue is ex- 

 tremely useful to man for mastication and 

 speech it is natural that the delicacy early de- 



