Page:Insects - Their Ways and Means of Living.djvu/312



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 atone 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 rime. In each species, the deviations flore 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 lift 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 bas 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 lit'e. The characters of the adult are, of course, impressed upon the germ cells and lnUSt be carried over to the next generation through the embryo, but they can not be developed at the same rime that the larval organs are functional. Cnse- quently, the cells, that are to form the special tissues of the adult remain through the larval period as small

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