External and Internal Factors in Evolution 335 



ulus acts for a long time, and through a large number of 

 generations, then it may, even if of small strength, so change 

 the idioplasm, that a tendency or disposition capable of being 

 seen may be the result. This appears to be the case in 

 regard to the action of light, which causes certain parts of 

 the plant to turn toward it and others away from it ; also 

 for the action of gravity, which determines the downward 

 direction of the roots. It may be claimed, perhaps, that 

 these are the results of direct influence and not of an 

 internal response, but this is not the case ; for some plants 

 act in exactly the opposite way, and send a stem downward, 

 as in the case of the cleistogamous flowers of Cardamine 

 cJienopodifolia ; and other plants turn away from the light. 

 This means that the idioplasm behaves differently in different 

 plants in response to the same stimulus. 



Concerning the more visible effects of adaptation, Nageli 

 states that in regard to some of them there can be no ques- 

 tion as to how they must have arisen. Protection against 

 cold, by the formation of a thick coat of hair, is the direct 

 result of the action of the cold on the skin of the animal. 

 The different weapons of offence and of defence, horns, 

 spurs, tusks, etc., have arisen, he maintains, through stimulus 

 to those parts of the body where these structures arise. 



The causes of the other adaptations, especially of those 

 occurring in plants, are less obvious. Land plants protect 

 themselves from drying by forming a layer of cork over the 

 surface. The most primitive plants were water plants, which 

 acclimated themselves little by little to moist, and then to dry, 

 air. When they first emerged from the water the drying 

 acted as a stimulus on the surface, and caused it to harden 

 in the same way as a drop of glue hardens. This harden- 

 ing in turn acted as a stimulus, causing a chemical transfor- 

 mation of the surface into a corky substance. This effect 

 was inherited, and in this way the power to form cork origi- 

 nated. 



