FACTS AND FACTORS OF DEVELOPMENT 5.37 



the germ cells, viz., (1) Constructive and destructive metabolism, 



(2) Reproduction, as shown in the division of cells and cell constituents, 



(3) Irritability, or the capacity of receiving and responding to stimuli. 

 All these general functions of living things are manifested by germ cells, 

 but as development advances each of these functions becomes more 

 specialized, more complicated and more perfect. A cell which at an 

 early stage was protective, locomotor and sensory in function may give 

 rise to daughter cells in which these functions are distributed to differ- 

 ent cells, cells which at an early stage were sensitive to many kinds of 

 stimuli give rise to daughter cells which are especially sensitive to one 

 particular kind of stimulus, such as vibration, light, or chemicals. 



Functions develop from a generalized to a specialized condition by 

 the process of " physiological division of labor " which accompanies mor- 

 phological division of substance. But just as in the development of 

 structures, new parts, which were not present in the germ, appear by a 

 process of "creative synthesis," so new functions appear in the course of 

 development, which are not merely sorted out of the general functions 

 present at the beginning, but which are created by the interaction and 

 synthesis of parts and functions previously present. 



Much less attention has been paid to the development of functions 

 than to the development of structures, and consequently it is not possible 

 to describe the former with the same degree of detail as the latter. But 

 in spite of the lack of detailed knowledge regarding the development 

 of particular functions the general fact of such development is well 

 established. To what extent structures may modify functions or func- 

 tions structures, in the course of development, is a problem which has 

 been much discussed, and upon the answer to which the fate of certain 

 important theories, for example Lamarckism, depends; but this problem 

 can be solved only by thorough-going experimental and analytical work. 

 In the meantime it seems safe to conclude that living structures and 

 functions are inseparable and that anything which modifies one of these 

 must of necessity modify the other also; they are merely different 

 aspects of organization, and are dealt with separately by the mor- 

 phologists and physiologists only as a matter of convenience. At the 

 same time there can be no doubt that minute changes of function can 

 frequently be detected where no corresponding change of structure can 

 be seen, but this shows only that physiological tests may be more deli- 

 cate than morphological ones. In certain lines of modern biological 

 work, such as bacteriology, cytology, genetics, many functional distinc- 

 tions are recognizable between organisms which are morphologically in- 

 distinguishable. But this does not signify that functional changes pre- 

 cede structural ones, but only that the latter are more difficult to see 

 than the former. For every change of function it is probable that an 

 "unlimited microscopist " could discover a corresponding change of 

 structure. 



vol. lxxxiv. — 37. (To be concluded.) 



