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 zdzoplasm, 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 
chenopodifolia ; 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. 
