ZOOLOGY SINCE DAKWIN. 481 



phases of development, nor in the circumstance that organs of developed 

 animals having similar structure and formation arise from different 

 germinal layers, but in the fact that the primary germinal layers arise 

 not only in different related animal stocks, but sometimes even within 

 one and the same phylum, in such a diverse manner that it would 

 certainly be a reversal of the old conception of homology 1 if, in spite of 

 this, they should be considered as morphological equivalents. 



The morphological point of view, which has hitherto exclusively pre- 

 vailed, will hardly serve us in this case. Experimental inquiry must 

 first, tracing out the causes of the different kinds of development, 

 seek to make clear to us "the internal mechanism of the phenomena." 

 Only in this way can we succeed in properly separating the "kenoge- 

 netic," or secondary, deceptive changes of form from the "palingenetic" 

 characters that represent originally inherited, developmental phases. 2 

 And until a more solid foundation for this shall be obtained, the theory 

 of the germinal layers, that has often been advanced with so much 

 confidence, can not be authoritatively formulated. 



Of double value, therefore, appear the facts that have been contrib- 

 uted to morphology from another phylogenetic source, that of the 

 natural history of extinct forms. Once an accessory of geology, this 

 has, during the period that we are considering, been developed into an 

 independent branch of study, and has borne richer and richer fruit in 

 proportion as it has recognized the necessity of correlating its advances 

 with the knowledge of living animals. 3 



Zoology and paleozoology have both the same chief aim — the elucida- 

 tion of the history of the animal forms of our globe, and a collection 

 corresponding to modern views, which require a complete representa- 

 tion of the material available for reconstructing the history of the race, 

 ought to place, side by side with the forms of animals now living, the 

 petrified remains of their ancestors. The generalized types, transition 



! C. Gegenbaur, the master of comparative anatomy, describes homology (special 

 homology) as " the relation between two organs that had a common origin; that is 

 to say, have been developed from a similar primitive form," and a complete homol- 

 ogy exists "whenever the organ in question, however it may be modified in shape, 

 size, and other relations, has constantly maintained its position and connections 

 unchanged." (Grundziige der vergleichenden Anatomie, 2 Auii., pp. 80-81, Leipzig, 

 1870.) 



C E. Haeckel, "Die Gastrula unddie Eifurchung derThiere" (Cap., Die Bedeutung 

 der Palingenie und der Cenogenie). (Jenaische Zeitschrift fur Naturwissenschaft, 

 IX. Bd., Jena, 1875.) 



C K. v. Zittel has been especially active in conducting paleontology into this new 

 path. By his teaching and researches, as well as by the exemplary arrangement of 

 the state paleontological collection in Munich, V. Zittel has continually worked 

 to secure the intimate cooperation of paleozoology and zoology, and his well-known 

 "Grundziige der Pahiontologie " (Miinchen und Leipzig, 1895) is directed to the 

 same end. Compare also K. v. Zittel, " Die Palliontologie und das biogenetische 

 Grundgesetz," in " Aula," Wochenblatt fiir die Akadem. Welt, I. Jahrg., page 385 

 et seq. Miinchen, 1895; as well as F. v. Wagner's review in the Biologische Central- 

 blatt, XL Bd., p. 840 et seq., Leipzig, 1895. 

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