306 Relation of Use and Disuse to Evolution 



So far as Lamarck was concerned, there was apparently no 

 intention here to call upon supernatural intervention or 

 upon unknown principles and forces. It is sufficient to 

 consider merely the common-sense implications of the fact 

 that living things do really respond to external stimuli and 

 to changed conditions in a more or less adequate manner. 



The most frequently cited examples of the operation 

 of this principle are those of the giraffe, of water birds, and 

 of the snake. In the absence of grass and herbs a grazing 

 animal will reach toward the leaves of the trees, and in 

 stretching for food will gradually lengthen his front legs 

 and his neck. A bird near the water, unable to find suffi- 

 cient food on the land, will wade into the shallows and either 

 attempt to swim, stretching the toes apart, or attempt to 

 keep its body out of the water, stretching its legs. The 

 obvious effect would be the formation of webbed feet in one 

 case and of long legs in the other. A four-legged lizard, 

 crawling through tight places and into narrow holes, will 

 gradually slough the legs and lengthen the body into a snake- 

 like form. It may be difficult to consider these examples 

 without having them seem absurd; but this is only because 

 many of us happen to know that the activities and *' needs " 

 of these hypothetical cases do not work out that way. 

 Giraffes do reach the leaves in the trees, but those fed on 

 hay from the ground get to be just as tall. Hens do not 

 acquire webbed feet or long legs when placed in water, even 

 when thrown into the water fresh from the egg. Lizards 

 do not become more snake-like when raised in the neighbor- 

 hood of holes or close vegetation. Animals do, however, 

 make in new situations responses that are adaptive — so ef- 

 fective, in fact, that one cannot but be impressed with their 

 seeming intelligence or purposiveness. A description of such 

 an adaptive response does not then seem so absurd. 



We may take by way of illustration the fact that an 

 animal infected with the germs of diphtheria and becoming 

 sick may, instead of succumbing to the disease, recover and 

 then remain indefinitely immune to further attacks. What 



