234 Animal Life and Intelligence. 



influences which their products, the rods and cones, have 

 become fitted to receive. I am not at present prepared to ac- 

 ceptthis theory of the germinal origin of all tissue-variations. 



Whether use and disuse are to be regarded as sources 

 of origin of variations is, again, a matter in which there 

 is wide difference of opinion. But if we admit that any 

 variations can take their origin in the body (as dis- 

 tinguished from the germ), then there is no a priori reason 

 for rejecting use and disuse as factors. As such, we are, I 

 think, justified, in the present state of our knowledge, in 

 reckoning them, at all events, provisionally. 



It is clear, however, that they are a proximate, not an 

 ultimate, source of origin. I mean that the structures 

 must be there before they can be either strengthened or 

 weakened by use or disuse. They are at most a source of 

 positive or negative variations of existing structures. They 

 cannot be a direct source of origin of superficial variations. 

 Gain or loss of colour ; form-variations not correlated with 

 organic variations ; — these cannot be directly due to use or 

 disuse. It is in the nervous and muscular systems and the 

 glandular organs that use and disuse are mainly operative. 

 When, however, organs are brought into relation, or fail to 

 be brought into relation, to their appropriate stimuli, we 

 speak of this, too, as use and disuse. We say, for example, 

 that persistent disuse may impair the essential tissues of 

 the recipient end-organs of the special senses, implying 

 that these tissues require to be brought into continued 

 relation to the appropriate stimuli in order that their 

 efficiency be maintained. So, too, we say that the epidermis 

 is thickened by use, meaning that it is brought into rela- 

 tion with certain mechanical stresses. Through correlation, 

 too, the effects of use and disuse may be widespread. Thus 

 increase in the size of a group of muscles may be correlated 

 with increase in the size of the bones to which they are in 

 relation. In fact, so knit together and co-ordinated is the 

 organism into a unity, it is probable that hardly any 

 variation could take place through use or disuse without 

 modifying to some extent the whole organic being. 



