VIL] SPECIES AND SPACE. 171 



for thinking that a southern continent, rich in living- 

 forms, once existed. Among tliese reasons is the distri- 

 bution of struthious birds around the antarctic resjion : as 

 the ostrich in Africa, the rhea in South America, the emeu 

 in Australia, the apteryx, dinornis, &c. in ^N'ew Zealand, 

 the epiornis in IMadagascar. Still the existence of such a 

 land would not alone explain the various geographical 

 cross relations which have been given above. It would 

 not, for example, account for the resemblance between the 

 Crustacea or fishes of New Zealand and of England. It 

 would, however, go far to explain the identity (specific or 

 generic) between fresh-water and other forms now simul- 

 taneously existing in Australia and South America, or in 

 either or both of these, and New Zealand. 



Again, mutations of elevation small and gradual (but 

 frequent and intermitting), through enormous periods of 

 time — waves, as it were, of land rolling many times in 

 many directions — might be made to explain numerous 

 difficulties as to geographical distribution, and any cases 

 that remained would probably be capable of explanation, 

 as being isolated but allied animal forms, now separated 

 indeed, but being merely remnants of extensive groups 

 which, at an earlier period, were spread over the surface 

 of the earth. Thus none of the facts here given are any 

 serious difficulty to the doctrine of "evolution;" but it is 

 contended in this book that if other considerations render 

 it improbable that the manifestation of the successive forms 

 of life has been brought abcut by minute, indefinite, and 

 fortuitous variations, then these facts as to geograpliical 

 distribution intensify that improbability, and are so far 

 worthy of attention. 



All geographical difficulties of the kind would be evaded 



