63 11. PERMANENCE OF SPECIES. 



effected in the plants are simply the natural results of 

 those conditions to which he subjects them. There 

 seems no reason to suppose that the same result which 

 man brings about more rapidly, could not have been 

 brought about, though more gradually brought about, 

 without his interference. If man can produce hereditarj^ 

 varieties of i)lants, which remain permanently different 

 from their central types, under his care, why cannot na- 

 ture also produce such hereditary varieties ? And in 

 what respect does an hereditary variety, the origin of 

 which is unknown, differ from a species ? * * * 



" On the whole, however, we seem to be justified in 

 asserting, that our knowledge of the present events in 

 nature, taken by itself, should incline us to a con- 

 clusion which is directly adverse to the theory of ' pro- 

 gressive development ' or ' transition of species ; ' yet 

 without affording us any actual disproof of that theory. 



" It is otherwise when our range of thought embraces 

 the vastly wider space of time, the events of which are 

 investigated by geologists. There we find ample evi- 

 dence to justify the conclusion that different species suc- 

 ceeded to each other. And no better mode of accounting 

 for this succession has been suggested, than the hj'pothe- 

 sis that one species passed into another, under changing 

 external conditions. Supposing this transition of species 

 to have taken place very gradually, and through a very 

 long series of descents, it would not require more rapid 

 change (from central types into varieties, and from a less 

 variety into a greater) than we see actually occurring in 

 the production of varieties at the present period of the 

 earth's history. 



" Could we ascertain that some varieties will continue 

 to vary from their central type, through many successive 

 descents ; and that, as they become less similar to their 



