360 THE EVOLUTION OF MAN. 



(Arthropoda), the Soft-bodied Animals (Mollusca), and the 

 Star Animals (Echinoderma), and we may, therefore, infer 

 that they have all acquired these, as a common inheritance 

 from the Ccelomati ; but we now meet with a passive 

 apparatus of movement, the skeleton system, which, in this 

 form, is exclusively peculiar to Vertebrates. Only the very 

 first rudiment of this, the simple notochord, is found in 

 Ascidia, which are the nearest invertebrate blood-relations 

 of Vertebrates. We infer from this, that the common 

 ancestors of both, the Chorda Animals, did not branch off 

 from the Worms till a comparatively late period. The 

 notochord is, it is true, one of those organs which appear at 

 a very early period in the vertebrate embryo ; but this is 

 clearly due to an ontogenetic heterochronism, to displace- 

 ment in time in the germ-history, that is, a gradual dis- 

 arrangement in the original phylogenetic secpicnce, caused 

 by embryonic adaptation. On Comparative Anatomical 

 grounds it may safely be assumed, that the first origin of 

 the skeleton system did not precede, but followed that of 

 the kidney system and of the vascular system, although 

 Ontogeny appears to indicate the contrary. 



Last of all the organ-systems, the sexual system finally 

 developed, in the sixth place, in our ancestors ; of course it 

 must be understood that this was last, in the sense that the 

 sexual apparatus acquired the independent form of a special 

 organ-system subsequently to all the other organs. The 

 simplest form, that of reproductive cells, is certainly very 

 ancient. Not only the lowest Worms and Plant Animals 

 propagate sexually, but this was also probably the case in 

 the common parent-form of all Metazoa, in the Gastnea; 

 but in all these low animals, the reproductive cells do not 



