176 GENERAL DISCUSSION [ch. xiv 



another, with but slight mutual interference, the latter being 

 quite possibly greater in animals. The evolution provides the 

 structurally different forms of life, while natural selection works 

 upon the functional side, and adapts them in detail for their 

 places in the local biological economy. There is no obvious reason 

 why selection should not develop small structural variations, 

 though one will not expect specific changes, unless rarely. In 

 general, selection will simply kill out those individuals, whether 

 new species or not, that commence anywhere with functional 

 characters that are unsuited to the conditions of the moment, or 

 that simply have ill-luck. Each new species, by mere heredity, 

 will probably have functional characters more or less closely 

 suited to the place in which it arises, but as time goes on, and the 

 number of species increases, chiefly by arrivals from elsewhere, 

 more and more careful adjustment will be needed to fit in each 

 newcomer. It is in this work that natural selection is of the first 

 importance, doing work that nothing else could do with the same 

 efficiency. 



Whether evolution must go on in all circumstances, we do not 

 know, for there is evidence like that of the widespread Hippuris 

 that seems to show that it is not perhaps absolutely necessary. 

 The evidence of the Podostemaceae seems to show that it may go 

 on without change of conditions, though perhaps only under the 

 action of a permanent force. If a plant suddenly arise with a 

 suitability to any particular mode of life, like a climber or a 

 parasite, natural selection will not kill it out, and it may go on 

 living, and perhaps do very well. 



As things show more and more definite adaptation to some 

 peculiarity of the conditions, they come up sooner and sooner in 

 their distribution against actual barriers to further spread, so 

 that they tend to occupy lesser areas than older and less adapted 

 species, perhaps closely related to them. As we have seen, a 

 species may become adapted to many regions, one by one (p. 145) 

 as it travels through them, but it need not show this adaptation 

 in external characters, nor have we any reason to suppose that 

 when it has become suited to B it remains necessarily suited to A. 

 It is possible that this functional adaptation, with or without 

 isolation, may result in genie changes that may be added up until 

 they cause a structural mutation. Admittedly we have not yet 

 solved the whole problem of adaptation as one may see it in so 

 many characters, but there is no evidence for the gradual adapta- 

 tion in structural characters that is demanded by natural selection. 



