O. HERTWIG 355 



blastula-like or gastrula-like ancestral form. We recognise 

 rather in the successive early stages of animal development 

 only the manifestation of special laws, by which the shaping 

 of animal forms (as distinct from plant forms) is brought 

 about" (p. 178, 1906, b). 



" The principal reason why certain stages recur in onto- 

 f geny with such constancy and always in essentially the same 

 manner is that they provide under all circumstances the 

 necessary pre-conditions through which alone the later and 

 higher stages of ontogeny can be realised. The unicellular 

 organism can by its very nature transform itself into a 

 I multicellular organism only by the method of cell-division. 

 C Hence, in all Metazoa, ontogeny must start with a segmen- 

 / tation-process, and a similar statement could be made with 

 (^regard to all the later stages" (p. 57, 1906, a). 



Similarities in early development are therefore no evidence 

 of common descent, and in the same way the resemblances of 

 adult animals, subsumed under the concepts of homology 

 and the unity of plan, are not necessarily due to community 

 of descent, but may also be brought about by the similarity 

 or identity of the laws which govern the evolution of these 

 animals. In the absence, therefore, of positive evidence as 

 to the actual lines of descent (to be obtained only from 

 palaeontology), homological resemblance cannot be taken as 

 proof of blood relationship, for homology is a wider concept 

 than homogeny. The only valid definition of homology is 

 that adopted in pre-evolutionary days, when those organs 

 were considered homologous " which agree up to a certain 

 point in structure and composition, in position, arrangement, 

 and relation to the neighbouring organs, and accordingly 

 possess identical functions and uses in the organism " ( p. 151, 

 1906, b). 



The concept of homology has thus a value quite 

 independent of any evolutionary interpretation which may 

 be superadded to it. " Homology is a mental concept 

 obtained by comparison, which under all circumstances 

 retains its validity, whether the homology finds its explana- 

 tion in common descent or in the common laws that rule 

 organic development" (p. 151, 1906, b). As A. Braun long 

 ago pointed out, " It is not descent which decides in matters 



