44 



GENERAL PRINCIPLES OF ZOOLOGY. 



their embryology (Fig. 6). Here it is shown that they 

 pass through a nauplius-stage (Fig. 6, a) characteristic of 

 most Crustacea, and that they then assume the shape of 

 completely formed small crabs (Fig. 6, /?), like those under 

 the name of Cyclops (Fig. 8, A), so widely distributed in fresh 

 waters. Very often the males make a halt in the cyclops- 

 stage and the female develops farther and becomes the 

 shapeless mass, so that there arises a very remarkable sexual 

 dimorphism (Fig. 7). All these examples, which can be 

 multiplied by hundreds, can be 

 explained in the same way. The 

 more developed forms pass 

 through the stage of organization 

 of the less developed, because 

 they spring from ancestors which 

 were more or less similar to the 

 latter. Man in his embryological 

 development passes through the 

 fish stage, the frog the perenni- 

 branchiate stage, the parasitic crab 

 first the nauplius- and then the 

 cyclops-stage, because their an- 

 cestors were once fish-like, peren- 

 nibranchiate-like, nauplius- and 

 cyclops-like. Here is expressed a 

 general phenomenon which Haeckel has stated in a general 

 proposition under the name of " the fundamental law of 

 Biogenesis." "The developmental history (ontogeny) of 

 an individual animal briefly recapitulates the history of the 

 race (phylogeny) ; i.e., the most important stages of organ- 

 ization which its ancestors have passed through appear again, 

 even if somewhat modified, in the development of individ- 

 ual animals." 



Examples of the Application of this Law. The Ner- 

 vous System. The fundamental law of biogenesis applies as 

 well to single organs as to the entire animals. The central 

 nervous system of the lower animals (echinoderms, ccelen- 

 terates, many worms) forms part of the skin; in its first 



* 



FIG. -j.Phiticthys Xiphite. a, fe- 

 male (after Claus), X 4 ; i>, male 

 (after Bergsoe), X 13. 



