THE GENERAL CONDITIONS OF LIFE 315- 



mental stages before they come to resemble the mother from which 

 they are derived. 



Since ancestors transmit their characteristics to their de- 

 scendants, these developmental stages become of extraordinary 

 importance in gaining a knowledge of the ancestral series ; for r 

 since they represent, in gross, forms inherited from ancestors, 

 they indicate, although in rude outline only, the developmental 

 forms that have once appeared in succession in the ancestral 

 series. In other words, the forms that appear in the germinal 

 development or ontogeny of an individual recapitulate in 

 gross the series of forms of the ancestors of the organism 

 in question. This fundamental law of biogenesis, which 

 was founded by Haeckel, and which has been discussed in 

 detail elsewhere, 1 enables us, by means of a critical examination 

 of the ontogenetic development of an organism, to reconstruct to a 

 certain degree its phylogenetic descent. 



From all these facts of palaeontology, comparative anatomy,, 

 and embryology for the full appreciation of which reference 

 must be had to the works of Darwin, Gegenbaur, Haeckel and 

 their pupils, which have laid the foundation for an understanding 

 of them the conclusion must necessarily be drawn that existing 

 organisms are derived in uninterrupted descent from the first 

 living substance that originated from lifeless substance. Moreover, 

 at the same time the path is indicated that has been taken by living 

 substance in its development upon the earth. The phylogenetic 

 research of modern morphology has succeeded in discovering this 

 path in general, and thus reconstructing in its gross outlines the 

 genealogical tree of organisms. Although much opposition was 

 expressed at first to the provisional scheme of genealogy that 

 Haeckel presented thirty years ago as an induction from the facts 

 then known, there are few morphologists now who do not 

 accept Haeckel's idea in its essential points. There now 

 prevails essential agreement regarding the phylogenetic relations 

 of the large groups of organisms, although, as to the smaller 

 groups and the special relations, many far-reaching differences of 

 opinion still exist j the latter will be set aside only gradually and by 

 new discoveries. In accordance with Haeckel's ideas and upon the 

 basis of the present condition of its knowledge, modern morpho- 

 logy has pictured somewhat as follows the genealogical tree of 

 organisms : 



1 Cf. P . 207. 



