PLASTICITY OF ORGANISMS 69 



to the individual species. Serious difficulties would however inevitably 

 arise if the qualitative and quantitative development of the several 

 tissues and mechanisms were absolutely constant for a given species, 

 and if, consequently, the scope and intensity of the various physiological 

 functions were precisely alike in every individual; since, namely, the 

 environmental conditions are subject to considerable variation within 

 the geographical range of any species, rigid adaptation to a particular 

 set of conditions would necessarily expose certain individuals to grave 

 risks, and might in certain circumstances be absolutely fatal. The 

 hereditary characters of a species must therefore be not only adaptive 

 hut also adaptable, within certain limits. In other words, they must 

 not be rigidly fixed, but must to quote Nageli possess a certain 

 plasticity, in order that the individual organism may be able to adjust 

 itself directly to the environmental conditions associated with its 

 particular habitat. Without some such property of self-regulation or 

 accommodation no organism could survive for any considerable length 

 of time. The senile decay and consequent gradual disappearance of 

 certain species seems to be due mainly as it is in the case of 

 individuals to decrease of plasticity on the part of the various 

 adaptive characters, or, in other words, to a diminished capacity for 

 physiological acc< immi tdation. 



The plastic quality of the vegetable organism finds expression in 

 the circumstance that the qualitative and quantitative development 

 of the several tissues are both to a certain extent dependent upon the 

 prevailing external conditions. In many cases, for example, the inten- 

 sity of illumination determines the degree of development of the 

 palisade-tissue. Similarly, differences in the humidity of the atmo- 

 sphere may call forth an adaptive modification of the epidermal walls 

 and of the ventilating system. Increased mechanical requirements 

 lead to an increased production of skeletal elements. In the present 

 state of physiological knowledge it is impossible to make any definite 

 statements concerning the chain of causation which, in such cases, 

 connects the external influence with the resultant internal modification: 

 it is safer to emphasise the complex nature of the correlation by regard- 

 ing the various environmental factors as stimuli, which may influence 

 the development of tissues to the advantage of the individual organism. 



C. THE ANATOMICO-PHYSIOLOGICAL CLASSIFICATION OF TISSUES. 



The term " tissue " ordinarily refers to some particular form or sort 

 of tissue. In its widest sense, it denotes any aggregation of cells 

 which have one or more characters in common : these characters may 

 he morphological, topographical, ontogenetic, or physiological in nature, 

 and the aims of the particular investigator will largely determine the 



