10 MORPHOLOGICAL DEVELOPMENT. 



velopment, sundry other general considerations should have 

 been included in the foregoing chapter when originally pub- 

 lished. This seems the most appropriate place for now nam- 

 ing them. Some were implicitly contained in the first vol- 

 ume, but it will be well definitely to state these, as well as 

 the others not yet implied. 



Interpretation of the forms of organisms and the forms 

 of their parts, must depend mainly on the conclusions pre- 

 viously drawn respecting their phylogeny; and the drawing 

 of such conclusions must be guided by recognition of the 

 various factors of Evolution, as well as by recognition of 

 certain extremely general results of Evolution and certain 

 concomitants of Evolution. 



A primary one among these is that no existing species can 

 exhibit more than approximately the ancestral structure of 

 any other existing species. As all ancestors have disappeared, 

 so, in a greater or less degree, the traits, specific, generic, or 

 ordinal, which distinguished the earlier of them have disap- 

 peared. Setting out with the familiar symbol, a tree, let us 

 regard its peripheral twigs as representing extant species; 

 let us assume that the interior of the tree is filled up with 

 some supporting substance, leaving only the ends of the 

 living twigs projecting; and let us suppose the trunk, main 

 branches, secondary branches, tertiary branches, &c., have 

 decayed away. Then if we take these decayed parts to stand 

 for the divergent and re-divergent lines of evolution which 

 are represented by fossils in the Earth's crust, it will be 

 manifest, first, that no one of the living superficial twigs (or 

 species) exhibits the ancestral organization whence any other 

 of the living superficial twigs (or species) has been developed; 

 it will be manifest, second, that the generic structure in- 

 herited by any existing species must be a structure out of 

 which came sundry allied species the fork, as it were, at 

 which adjacent twigs diverged; and third, that the ancestor 

 of an order must, in like manner, be sought at some point 

 deeper down in the symbolic tree a place of divergence of 



