380 THE EVOLUTION THEOEY 



tors and nearest relatives. Many parasitic Crustaceans, such as the 

 Rhizocephalids, lack almost all the typical characteristics of the 

 crustacean body, and dispense not only with segmentation, with 

 head and limbs, but also with stomach and intestine. As we have 

 seen, they feed like the lower fungi, by sucking up the juices of 

 their hosts, by means of root-like outgrowths from the place where 

 the mouth used to be. Nevertheless, their relationship to the Cirrhi- 

 pedes can be proved from their larval stages. There are, however, 

 parasites in the kidneys of cuttlefish — the Dicyemidse — in regard to 

 which naturalists are even now undecided whether they ought to 

 form a lowly class by themselves between unicellular animals and 

 Metazoa, or whether they have degenerated, by reason of their 

 parasitism, from the flat worms to a simplicity of structure elsewhere 

 unknown. They consist only of a few external cells, which enclose 

 a single large internal cell, possess no organs of any kind, neither 

 mouth nor intestine, neither nervous system nor special reproductive 

 organs. But although degeneration cannot be proved in this case, 

 it can be in hundreds of other cases with absolute certainty, as, 

 for instance, in the Crustacea belonging to the order of Copepods, 

 which are parasitic upon fishes, in which we find all possible stages 

 of degeneration, according to the degree of parasitism, that is, to 

 the greater or less degree of dependence upon the host ; for organs 

 degenerate and disappear in exact proportion to the need for them, 

 and they thus show us that degeneration also is under the domination 

 of adaptation. 



Thus retrogressive evolution also is based upon the power of the 

 living units to respond to changing influences by variation, and upon 

 the survival of the fittest. 



The roots of all the transformations of organisms, then, lie in 

 changes of external conditions. Let us suppose for a moment that 

 these might have remained absolutely alike from the epoch of 

 spontaneous generation onwards, then no variation of any kind and 

 no evolution would have taken place. But as this is inconceivable, 

 since even the mere growth of the first living substance must have 

 exposed the different kinds of biophors composing it to difterent 

 influences, variation was inevitable, and so also was its result — the 

 evolution of an animate world of organisms. 



External influences had a twofold effect at every stage upon 

 every grade of vital unit, namely, that of directly causing variation 

 and that of selecting or eliminating. Not only the biophors, but 

 every stage of their combinations, the histological elements, chloro- 

 phyll bodies, muscle-disks, cells, organs, individuals, and colonies, can 

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