LOSS AND CHANGE OF FUNCTION 3 



tion of the stomata and the loss of function on the part of their 

 guard-cells are secondary consequences of the phylogenetic reduction or 

 rather of the complete atrophy which the photosynthetie system has 

 undergone in the sporogonium ; owing to the disappearance of green 

 tissue, the development of special ventilating organs in the shape of 

 stomata has become superfluous. 



Ontogenetic loss of function naturally cannot be regarded as a 

 phenomenon of prime importance from the standpoint of physiological 

 anatomy. When, namely, a particular organ or tissue ceases its 

 physiological activity at a relatively early stage, as compared with 

 other tissue-systems, it must do so for reasons which are in some way 

 or other connected with its proper functions. The fact that loss of 

 function may arise gradually in the course of evolution also in no 

 wise affects the fundamental principles of the anatomico-physiological 

 method. An organ or tissue which has undergone phylogenetic reduc- 

 tion must ex hypothesi have performed a definite function at some 

 previous evolutionary stage, and must at that time have possessed an 

 anatomical structure correlated with that function. Where reduction 

 has not progressed too far, this former connection between structure 

 and function may still be demonstrable. In cases of extreme reduction 

 however, the existing morphological features may no longer be capable 

 of interpretation in terms of their obsolete physiological value. In 

 such instances it must always remain uncertain whether the reduced 

 structure ever did perform any special function. Tims the gritty 

 particles, composed of thick-walled stereides, which occur in the flesh 

 of the Common Pear (and in other Pomaceae), have been regarded as 

 relics of an ancestral protective shell. This interpretation may be 

 quite correct ; but the thick-walled character of these stone-cells is not 

 in itself a sufficient proof of their former mechanical function. For 

 there is just as much to be said in favour of the view according to 

 which these thick-walled stone-cells are devoid of any functional 

 significance, and merely serve as repositories for superfluous cellulose 

 material. Such ambiguous cases are probably rare. 



Functionless characters sometimes arise in consequence of change 

 of function, in which event they owe their existence partly to the 

 influence of heredity. Numerous cases will be met with later on, in 

 which the principal function of a given organ or tissue becomes altered 

 in the course of phylogenetic or it may be of ontogenetic develop- 

 ment, the change being accompanied by an appropriate modification 

 of structure. It does not follow, however, that all the morphological 

 features correlated with the original function necessarily undergo 

 alteration in such cases. On the contrary, characters which are not 

 incompatible with the newly acquired function are often inherited 



