1^1 LIBRARY 



CHAPTER I 



PROBLEMS AND MATERIAL 



DEVELOPMENT of the individual organism is a continuous series 

 of events in space and time. At any period of development the 

 order is evident as a definite system of activities and structures 

 which constitute the pattern of the individual at that period. The indi- 

 vidual is a realization in these activities and structures of potentialities of 

 the protoplasm concerned in relation to conditions external to it, and these 

 external conditions are essential factors in determining what potentialities 

 are realized in any particular case. When we subject developing organ- 

 isms to experimental conditions, the course of development may be modi- 

 fied and the resulting organism may be widely different from that char- 

 acteristic of the species under natural conditions. These modifications 

 represent realization of other potentialities. Evidently the so-called "nor- 

 mal" individual is a realization of only a part of the potentialities of the 

 species-protoplasm. Since development of different organisms takes place 

 in protoplasms which differ specifically in constitution and possess differ- 

 ent potentialities, the individual structural and functional patterns which 

 develop also differ; and on the basis of these differences we distinguish 

 species, genera, and larger groups. 



The individual organism is a physiologically integrated unit — a whole 

 with actual physiological relations between its parts, although at certain 

 developmental stages a greater or less degree of independence of some 

 parts as regards further differentiation may occur. In short, it seems nec- 

 essary to conceive development as essentially a series of events integrated 

 into a definite order, pattern, and unity which differs in character with the 

 constitution of the protoplasm in which development is taking place. 

 What problems does this phenomenon of development present? To ask 

 this question is to ask what the problems of biology are, for life is develop- 

 ment. And if we attempt to distinguish certain problems as fundamental, 

 we are in no better case, for, unless we are content with the mere accumu- 

 lation of facts, we cannot proceed far in the study of development from 

 any point of attack without coming face to face with the great problems 

 of biology. 



