GEOLOGICAL SUCCESSION OF ORGANIC BEINGS 127 



ness, under such extremely complex relations, the stand- 

 ard of organization of the imperfectly-known faunas of 

 successive periods. 



We shall appreciate this difficulty more clearly by 

 looking to certain existing faunas and floras. From the 

 extraordinary manner in which European productions have 

 recently spread over New Zealand, and have seized on 

 places which must have been previously occupied by the 

 indigenes, we must believe, that if all the animals and 

 plants of Great Britam were set free in New Zealand, 

 a multitude of British forms would in the course of time 

 become thoroughly naturalized there, and would extermi- 

 nate many of the natives. On the other hand, from the 

 fact ^hat hardly a single inhabitant of the southern hemi- 

 sphere has become wild in any part of Europe, we may 

 well doubt whether, if all the productions of New Zea- 

 land were set free in Great Britain, any consideral^le 

 number would be enabled to seize on places now occu- 

 pied by our native plants and animals. Under this point 

 of view, the productions of Great Britain stand much 

 higher in the scale than those of New Zealand. Yet 

 the most skilful naturalist, from an examination of the 

 species of the two countries, could not have foreseen 

 this result. 



Agassiz and several other highly competent judges 

 insist that ancient animals resemble to a certain extent 

 the embryos of recent animals belonging to the same 

 classes; and that the geological succession of extinct 

 forms is nearly parallel with the embryological devel- 

 opment of existing forms. This view accords admirably 

 well with our theory. In a future chapter I shall at- 

 tempt to show that the adult differs from its embryo, 



