APPENDIX D 219 



differentiates high animal organisms from low animal organisms. 

 Without this power and the plasticity which results from it, the 

 multitudinous parts of high animals could not well be co- 

 ordinated, and, therefore, without it their evolution could scarcely 

 have been possible. Indeed, it is not much to say, so vitally 

 important is this power to the higher animals, that, as regards 

 them, the chief aim (if I may use the expression) of Natural 

 Selection has been to evolve it. But, since this power of 

 developing in response to the stimulation of use operates mainly 

 along certain definite Hnes, which are not quite the same in 

 every species, the different species differ as regards size and 

 shape, not only in characters which are inborn, but also in those 

 which are acquired. Thus an ox differs in size and shape from 

 a man not alone in inborn characters, but also in characters 

 which are acquired as a result of exercise and use. The structures 

 of both the ox and the man develop in response to appropriate 

 stimulation, but not quite in the same direction, nor in the same 

 proportion, nor to the same degree ; hence, to some extent the 

 differences in size and shape betwixt the two animals. Consider, 

 for instance, the hind limbs of the ox and man : in both these 

 grow greatly in response to the stimulation of exercise, but the 

 lines of growth being somewhat different, the limbs do not 

 approximate in shape and size. Presently, when we consider 

 mind, we shall realise even more strikingly the importance of 

 our subject, and perceive how deeply it concerns many fields of 

 thought and investigation which have greatly interested mankind 

 in all ages ; but I have still something more to say as regards 

 physical characters, though it is not possible in the space allotted 

 to me to do full justice to the theme. 



Acquired physical characters (properly so termed) may involve 

 not only quantitive changes, which alone we have as yet con- 

 sidered, but qualitative changes also. Here, again, a wild field 

 for investigation presents itself. For example, in man exercise 

 does not merely cause a muscle to increase in size ; it occasions 

 besides, as in athletes after training, an increase in efficiency 

 if.e. in the power and duration of contraction) which is greatly 



