366 STATE OF DEVELOPMENT 0^ 



at this increase in number in the highest class, which im- 

 plies a great displacement of lower forms, as a decided 

 advance in the organization of the world. We thus see 

 how hopelessly difficult it is to compare with perfect fair- 

 ness, under such extremely complex relations, the standard 

 of organization of the imperfectly- known faunas of succes- 

 sive periods. 



We shall appreciate this difficulty more clearly by look- 

 ing to certain existing faunas and floras. From the extra- 

 ordinary 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 Britain were set free in New Zealand, a 

 multitude of British forms would in the course of time 

 become thoroughly naturalized there, and would exter- 

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

 fact that 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 Zealand 

 were set free in Great Britain, any considerable number 

 would be enabled to seize on places now occupied 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 skillful 

 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 em- 

 bryos of recent animals belonging to the same classes; and 

 that the geological succession of extinct forms is nearly 

 parallel with the embryological development of existing 

 forms. This view accords admirably well with our theory. 

 In a future chapter I shall attempt to show that the adult 

 differs from its embryo, owing to variations having super- 

 vened at a not early age, and having been inherited at a 

 corresponding age. This process, while it leaves the em- 

 bryo almost unaltered, continually adds, in the course of 

 successive generations, more and more difference to the 

 adult. Thus the embryo comes to be left as a sort of 

 picture, preserved by nature, of the former and less modi- 

 fied condition of the species. This view may be true, and 



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