GUIDING FACTORS. INTRODUCTION 347 



Among segmental organisms we have another advance in bodily 

 specialization. Here chains of individuals, each, in a sense, morpho- 

 logically equivalent to a single individual of the flatworm type, be- 

 come welded together into more complex segmented organisms. With 

 this new level of complexity of organization comes the possibility of 

 specializing different segments, each of them equivalent to a whole 

 individual of the lower grade, for particular functions. Some per- 

 form chiefly nervous and sensory functions, others reproductive 

 functions, and still others respiratory or digestive functions. Each 

 segment in certain parts of the body becomes a specialist in some 

 one function which it performs more efficiently when relieved of some 

 of the other functions. Of course, specialization would be of no value 

 unless integration and interdependence of part upon part kept pace 

 with it. Hence, one of the results of the trends just described is the 

 closer integration of parts and the greater defrniteness of individuality. 



Even among segmental organisms, as for example the ants and 

 bees, division of labor takes place among segmental individuals, and 

 a new and higher level of organization — social organization — emerges. 

 Here in these insect societies, individuals are specialized in several 

 ways: some become functional males and females, others (immature 

 females) become specialized as workers or protective units known as 

 "soldiers." Thus, a society is, in a sense, a fairly definitely integrated 

 individual of a higher level, involving a high degree of interdependence 

 of the various kinds of specialists upon one another. The integrating 

 factor seems to be associated with the family relationship of all mem- 

 bers of a given colony. All members of a given colony are offspring 

 of the same mother and father. They are extremely clannish and 

 resent vigorously any intrusion by outsiders. 



It therefore appears that one of the principal progressive trends of 

 evolution is one in which units of lower orders are aggregated into 

 complex units of higher orders. Each aggregate is a new type of in- 

 dividual, with new properties not entirely the result of a summation 

 of the properties of the ingredient units. This idea has sometimes 

 been spoken of as emergent evolution, according to which, with each 

 higher level of units, new properties of the whole emerge which are 

 more than the sum of properties of the parts. This might also be 

 thought of as a kind of creation, for something new comes into being as 

 though out of nothing. 



While the foregoing discussion is, in a sense, a digression, it seems 

 well to call attention somewhere in a book on evolution to these facts 



