220 A STUDY IN HEREDITY 



out of proportion to the increase in size. Intermittent friction 

 or heat or other irritant {e.g. chemical) not merely causes the 

 skin to thicken, as in corns and callosities ; it renders it denser 

 also. Again, stimulation (that is use) may result in change which 

 is wholly qualitative. Thus eyes which, when unaccustomed to 

 the task, are rendered sore by the continued scrutiny of small 

 objects {e.g. print, as in the case of an adult learner) may by 

 practice be trained, without apparent physical change, to endure 

 this proceeding without damage. Most of these qualitative 

 changes are best studied in connection with mind. 



I have said that the power of acquiring physical traits does 

 not exist among lower animals, or, if it exists, does so in pro- 

 portion as they are lowly placed in the scale of life, to an extent 

 very small as compared to its development among high animals. 

 If I am right as to this, low animals {e.g. invertebrates) should 

 be incapable or little capable of acquiring immunity against 

 zymotic disease. I am not aware, however, that any observations 

 on the subject have been made. 



It is possible that many who read the foregoing will be 

 inclined to dispute the facts and inferences put forward, and to 

 urge, for instance, that I have not established any proof, nor 

 even brought forward convincing evidence, of the truth of my 

 assertion that low animals are incapable, or less capable than 

 high animals, of acquiring physical characters. There is, in 

 truth, no literature to which I can appeal, for the question is 

 entirely new ; and therefore, also, so far as I am aware, no 

 experiments directly bearing on it have been made. Moreover, 

 in the highest animals all acquired physical characters are 

 merely extensions of previously existing inborn characters. Thus 

 the limb of an infant, which is compounded, as we may suppose, 

 almost entirely of that which is inborn, grows under the influence 

 of exercise and use into an adult limb. There is a sharp dividing 

 line, but we cannot perceive it; and, therefore, as regards the 

 infant's limb, we cannot yet say where the inborn ends and the 

 acquired begins. But in mind, which we have next to consider, 



