90 ORGANIC EVOLUTION— THE FACTOKS 



remarkable degree (e.g. stimulated by exercise, the 

 muscles and glands increase in size and in vigour; under 

 intermittent pressure the skin thickens and hardens ; as 

 a consequence of the deep breathing caused by a venous 

 condition of the blood, the lungs expand) ; but while the 

 power of varying is clearly transmissible, there is nothing 

 to show that the acquired variations themselves, which 

 result from the exercise of it, are at all transmissible, 

 whereas there is a mass of negative evidence tending to 

 prove the converse; for instance, millions of people 

 acquire callosities in their hands and feet, but none ever 

 transmit them to their offspring — only the power to 

 acquire the callosities anew is transmitted. Now this 

 vitally important power, which can have arisen only by 

 the accumulation of inborn variations (for instance, the 

 accumulation of acquired variations can have had nothing 

 to do with the evolution of the power the organism 

 possesses of repairing a cut or abraded surface), is so great 

 that, in a high multicellular adult organism such as a 

 man, it is impossible at first sight to say what part of 

 the whole development is due to the accumulation of 

 inborn variations, and what to individually acquired and 

 non-transmissible variations. For instance, since the 

 muscles may be increased considerably in size if exercised, 

 and may practically disappear if not exercised, we cannot 

 say at first sight how much of the muscular development 

 of a man is due to direct inheritance, and how much to 

 exercise — i. e. to the power of varying to suit the cir- 

 cumstances, of developing under appropriate stimulation. 

 If the limb of an infant be paralyzed or so injured 

 that the joints are locked, then, in consequence of the 

 lack of stimulation, it (the limb) grows very little after- 

 wards ; muscles, bones, ligaments, blood-vessels, &c. all 

 remain dwarfed and undeveloped. Again, if an adult 

 limb be paralyzed, its component structures, no longer 

 maintained by the stimulation of use, atrophy in their 



