196 NATURAL SCIENCE. September. 



the theory of the homology of the germ-membranes. So surely as 

 the egg of all animals has the same value in form as a cell, and directs 

 us to unicellular Protozoa as the starting point of all higher organisms, 

 so surely also must the consequent embryonic phases of the multi- 

 cellular-tissue creatures (Metazoa) show signs of common ancestry. 

 Haeckel, starting from this premise, thought, in fact, that he could 

 find the image of that ancestral form common to all multicellular 

 organisms in the wide-spread evolutionary stage of the " Gastrula," 

 which consists of two concentric cell-layers and a rudimentary 

 (primitive) mouth. ^ These two cell-layers were determined to be 

 the same (homologous) in all forms, so that here there seemed to 

 be a means of tracing not only within the single lines of descent 

 (phyla), but also through the whole kingdom of the Metazoa, the 

 homology of the organs. 



But if we examine without prejudice the facts brought forward, 

 we must admit that this comprehensive attempt has as yet not been 

 successful. The deeper our knowledge of evolution becomes, and 

 the more exactly the comparison is undertaken, the higher also do 

 difficulties tower above us. The essential point, however, is not 

 that it requires a great stretch of imagination to refer all the 

 evolutionary stages mentioned here to the scheme of the Gastrula, 

 nor the circumstance that so often in adult animals organs, similar in 

 form and function, actually originate from different germ-membranes ; 

 it is the fact that the primal germ-membranes arise not only in the 

 members of different animal stocks, but occasionally even within one 

 and the same phylum, in such different ways that it would be turning 

 the old idea of homology ^ upside down if one yet regarded them as 

 morphologically equivalent. 



And here the hitherto exclusively morphological standpoint alone 

 will hardly be of use. Experimental investigation, while tracing the 

 causes of the various developed species, must first seek to make clear 

 the " inner mechanism of the phenomena of life." Only on these 

 lines can a secure basis be obtained for distinguishing between the 

 " cenogenetic " (secondary falsifications of types) and the " palinge- 

 netic " (representing the originally inherited developmental tendency) 

 characters. 3 And until a firmer foundation is worked out for this, a 



1 E. Haeckel, " Die Gastraea-Theorie, die phylogenetische Classification des 

 Thierreichs und die Homologie der Keimblatter." Jenaische Zeitschrift fi'ir Natur- 

 wissenschaft, VIII. Bd. Jena, 1874. 



- C. Gegenbaur, the Nestor of comparative anatomy, describes as homology 

 (special homology) " the relation between two organs of the same origin, and proceed- 

 ing from the same stock," and complete homology is present "when the aforesaid 

 oigan, even if modified in form, surroundings, and many other respects, has kept itself 

 quite unchanged in position and relationship." {Grunizi'ige der vergleicheuden Aitatomie, 

 2 ed. Pp. 80-Si. Leipzig, 1S70.) 



' E. Haeckel, " Die Gastrula und die Eifurchung der Thiere " (Chapter " Die 

 Bedeutung der Palingenie und der Cenogenie "). Jenaische Zcitschr. f. Natunviss., 

 IX. Bd. Jena, 1875. 



