94 ti^vtil 



and flora of the Steppes, owing to their gradual change to the desert 

 condition, with consequent enormous destruction of animal and vege- 

 table life. 



Speciew, then, which are being more and more restricted in range, 

 can be recognised by the fact that they become gradually rarer until 

 at last they disappear. Examples in Britain are unfortunately only 

 too common, especially among the butterflies. 



4. — Evolution of New Species. 



In all the eases just mentioned, in addition to the direct extinction 

 of the original habitant species and the introduction from without of 

 immigrant species better suited to the new conditions, there is the 

 possible transmutation of one form into another better fitted to cope 

 with the new environment. Our detailed knowledge of animal physi- 

 ology is not yet wide enough, and our historical record of it is far too 

 brief, to permit us to give definite proved examples of this most inter- 

 esting change, though it is evident that it must occur ; but we may 

 note that it is frequently urged that the progressive darkening of 

 many Lepidopterous species, e.g., the Peppered Moth (Blstoti betnla- 

 rius L.), is due to the new conditions brought about by the industrial 

 revolution in Britain and other countries, with the consequent pollu- 

 tion by soot of trees, fences, etc.* There is presumably nothing in- 

 herently improbable in this conjecture, although it has not been 

 definitely proved, nor does it meet with universal support. 



5.— Relation to other Organisms. 



Changes such as those indicated above, which involve the exist- 

 ence, the distribution, the numbers or the permanence of a vegetable 

 or animal form, must of necessity affect other organisms also which 

 are dependent on it for its means of existence; for example, organisms 

 which depend directly on other animals or plants as sources of food 

 must become extinct with the extinction of their food supply. This is 

 shown very clearly during the drainage of fens, the reclamation of 

 waste land, and the destruction of forests. But the same thing 

 applies also to ecto- and endo-parasites and to commensals ia all 

 degrees of dependence on their host or messmate, so that the distribu- 

 tion and numbers of the one must be closely correlated with those of 



* It is iiiiiuatei-ial for the purposes of the present paper whether thi.s inuLuiic teinleiicy i« an 

 entirely new eonditiou, or, as has been suggested for some speeies at least, a reversion to tyjje — 

 the idea of the adaptation of a species to its new environment still holds. 



