316 THE ORIGIN OF SPECIES 



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 man- 

 ner 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 exterminate 

 many of the natives. On the other hand, from the fact that hardly 

 a single inhabitant of the southern hemisphere has become wild 

 in any part of Europe, we may well doubt whether, if all the pro- 

 ductions 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 skilful naturalists, 

 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 embryo- 

 logical development of existing forms. This view accords ad- 

 mirably well with our theory. In a future chapter I shall attempt 

 to show that the adult differs from its embryo, owing to variations 

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

 at a corresponding age. This process, while it leaves the embryo 

 almost unaltered, continually adds, in the course of successive 

 generations, more and more difference to the adult. Thus the em- 

 bryo comes to be left as a sort of picture, preserved by nature, of 

 the former and less modified condition of the species. This view 

 may be true, and yet may never be capable of proof. Seeing, for 

 instance, that the oldest known mammals, reptiles, and fishes 

 strictly belong to their proper classes, though some of these old 

 forms are in a slight degree less distinct from each other than are 

 the typical members of the same groups at the present day, it 

 would be vain to look for animals having the common embryo- 

 logical character of the vertebrata, until beds rich in fossils are 

 discovered far beneath the lowest Cambrian strata — a discovery 

 of which the chance is small. 



