326 MENTAL EVOLUTION OF ANIMALS. 



newly-born mammals are able, without such individual ex- 

 perience as is required in the case of man, immediately and 

 correctly to supply all the mental inferences which are 

 needed to complete their sensuous perceptions. Of course 

 the explanation of this must be that heredity in these cases 

 has already done the work, so that the young animal comes 

 into the world with its mental endowments of perceptive 

 inference as fully elaborated and as completely efficient as its 

 bodily endowments of perceptive sensations. But the ques- 

 tion arises, Why is not this also the case with Man ? That 

 it is not the case is sufficiently proved by the results of 

 couching for cataract previously quoted ; but why it should 

 not be the case is not quite so clear, and hitherto has not been 

 sufficiently considered ; for it is only since the experiments of 

 Mr. Spalding that the facts with regard to animals have 

 become known.* I think the answer to this question is 

 as follows. 



First of all, there is no evidence to show that even in the 

 case of man heredity has not played a very important part 

 (though not so important as with animals) in supplying the 

 machinery of perceptive inference. Indeed I think we have 

 some evidence to show that it has ; for only by supposing 

 this are we able to explain why the youth whose case was so 

 well described by Mr. Cheselden was able, after so short an 

 interval as three months, to perceive the illusory effects of 

 shading and perspective in a picture. But, even if it be 

 allowed that heredity here played an important part, there is 

 still, no doubt, a great discrepancy to be explained in the 

 degree of its influence as compared with its absolute per- 

 fection in the case of the lower animals. But I think 

 there are two considerations which, taken together, are suffi- 

 cient to explain the discrepancy. In the first place we have 

 already seen, when treating of the hereditary instinctive 

 endowments of animals, that the machinery of these endow- 

 ments is apt to be thrown out of gear if they are not allowed 

 full play at the time of life when they ought normally to 

 have first come into operation. Therefore in the case of this 

 youth it seems highly probable that during the twelve years 



* Or rather, I should say, so well known. Houzeau had pointed out 

 that while young infants are unable to localize a pain or other sensation, 

 newly -bom calves are able to do so with precision (see Fac. Mem. des 

 Animaux, torn, i, p. 52). 



