486 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



species. Or, again, permanence may be compared to a 

 right cylinder standing on end. It may lean in any 

 direction to a limit and right itself, but pressed too far it 

 is overthrown, and the species is destroyed. Within these 

 limits species are made according to a plan or type at 

 once, and these continued by generation unchanged. 

 They are, as it were, struck from the same die, until the 

 die is broken or worn out and another made. Thus the 

 process goes on by an alternation of supernatural and 

 natural means. The origin was supernatural, the con- 

 tinuance by natural process of generation. The mak- 

 ing of dies was supernatural, the coinage was natural. 



This old theory, as already shown, explains many of 

 the phenomena given above, but not all. For example : 

 i. If each species were made especially for a certain 

 place and environment, then it ought to be more per- 

 fectly adapted to that place than any other, but, on the. 

 contrary, introduced species of ten flourish better in their neiv 

 than in their old homes. 2. Again, if species are made each 

 in its own place and spread as far as they can and wher- 

 ever they can— as indeed they do — then the amount of 

 difference between faunas of different places ought to 

 be in strict proportion to the impassableness of the bar- 

 riers between. This is indeed largely true, but not the 

 whole truth. There is another element which is left out — 

 viz., the element of time. The difference is proportioned 

 to the impassableness of the barrier and the time since the 

 barrer was set up. This element of time connects the 

 subject with the idea of evolution of organic forms 

 throughout all geological time. In a word, the old 

 theory was well enough for the present condition of 

 things, though not perfect even there, but fails entirely 

 to connect present faunas and their distribution with 

 those of previous times. The study of the present alone 

 is but a flash-light view of the world, and therefore the 



