INSECTS 



morphic transformation, and yet another to discover how 

 the change is accomplished in the individual. Meta- 

 morphosis can be only a special modification of general 

 developmental growth, and growth toward maturity by 

 the individual goes over the same field that the species 

 traversed in its evolution. Yet, the individual in its 

 development may depart widely from the path of its 

 ancestors. It may make many a detour to the right or 

 the left; it may speed up at one place and loiter along at 

 another; and, since the individual is rather an army of 

 cells than a single thing, certain groups of its cells may 

 forge ahead or go off on a bypath, while others lag behind 

 or stop for a rest. Only one condition is mandatory, and 

 this is that the whole army shall finally arrive at the same 

 point at the same time. In each species, the deviations 

 from the ancestral path, traveled for many generations, 

 have become themselves fixed and definite trails followed 

 by all individuals of the species. The development of 

 the individual, therefore, may thus come to be very 

 different from the evolutionary history of its species; and 

 the life history of an insect with complete metamorphosis 

 is but an extreme example of the complex course that 

 may result when a species leaves the path of direct de- 

 velopment to wander in the fields along the way. 



The larva and the adult insect have become in many 

 cases so divergent in structure, as a result of their separate 

 departures from the ancestral path, that the embryo has 

 become almost a double creature, comprising one set of 

 cells that develop directly into the organs of the embryo 

 and another set held in reserve to build up the adult 

 organs at the end of the larval life. The characters of 

 the adult are, of course, impressed upon the germ cells 

 and must be carried over to the next generation through 

 the embryo, but they can not be developed at the same 

 time that the larval organs are functional. Conse- 

 quently, the cells, that are to form the special tissues 

 of the adult remain through the larval period as small 



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