Many of our domesticated dogs, when we gently scratch their sides and certain other parts of the body, will themselves perform scratching movements with the hind leg of the same side as that upon which the irritation is being supplied. According to Goltz[46], this action is a true reflex; for he found that it is performed equally well in a dog which has been deprived of its cerebral hemispheres, and therefore of its normal volition. Again, according to Haycraft[47], this reflex is congenital, or not acquired during the life-time of each individual dog. Now, although the action of scratching is doubtless adaptive, it appears to me incredible that it could ever have become organized into a congenital reflex by natural selection. For, in order that it should, the scratching away fleas would require to have been a function of selective value. Yet, even if the irritation caused by fleas were supposed to be so far fatal in the struggle for existence, it is certain that they would always be scratched away by the conscious intelligence of each individual dog; and, therefore, that no advantage could be gained by organizing the action into a reflex. On the other hand, if acquired characters are ever in any degree transmitted, it is easy to understand how so frequently repeated an action should have become, in numberless generations of dogs, congenitally automatic. [46] _Pflüger's Archiv_, Bd. xx. s. 23 (1879). [47] _Brain_, part xlviii, pp. 516-19 (1889).--There is still better proof of this in the case of certain rodents. For instance, observing that rats and mice are under the necessity of very frequently scratching themselves with their hind-feet, I tried the experiment of removing the latter from newly-born individuals--i.e. before the animals were able to co-ordinate their movements, and therefore before they had ever even attempted to scratch themselves. Notwithstanding that they were thus destitute of individual experience with regard to the benefit of scratching, they began their scratching movements with their stumps as soon as they were capable of executing co-ordinated movements, and afterwards continued to do so till the end of their lives with as much vigour and frequency as unmutilated animals. Although the stumps could not reach the seats of irritation which were bent towards them, they used to move rapidly in the air for a time sufficient to have given the itching part a good scratch, had the feet been present--after which the animals would resume their sundry other avocations with apparent satisfaction. These facts showed the hereditary response to irritation by parasites to be so strong, that even a whole life-time's experience of its futility made no difference in the frequency or the vigour thereof.