PRESENT ASPECTS OF EVOLUTION 287 



more nutriment to the cells composing the lowest 

 layer of the outer skin or epidermis. These cells, be- 

 ing better nourished, reproduce by division more 

 rapidly, and the epidermis, becoming composed of a 

 greater number of layers of cells, thickens. The outer- 

 most layers, being farthest from the blood supply, dry 

 up and are packed together into a horny mass. 



If I go out into the sunshine I become tanned. 

 This again is not a direct and purely chemical or 

 physical result of the sun's rays, but these have 

 stimulated the cells of the skin to undergo certain 

 modifications. Any change in the living body under 

 changed conditions is not passive, but an active re- 

 action to a stimulus furnished by the surroundings. 

 The same stimulus may excite very different reactions 

 in different individuals or species. 



Early in this century a farmer, Seth Wright, foimd 

 among his lambs a young ram with short legs and 

 long body. The farmer kept the ram, reasoning that 

 his short legs would prevent him from leading the 

 flock over the farm-walls and fences. From this ram 

 was descended the breed of ancon, or otter, sheep. 

 Now the stimulus which had excited this variation 

 must have been applied early in embryonic life, or 

 perhaps during the formation or maturing of the germ- 

 cells themselves. Such a variation we call a con- 

 genital variation. 



These cases are merely illustrations of the general 

 truth that in every variation there are two factors 

 concerned : the Uving being with its constitution and 

 inherent tendencies and the external stimulus. 



The courses of the different balls in a charge of 

 grape-shot, hurled from a cannon, are evidently due 



