40 MANUAL OF ZOOLOGY. 



natural selection — propounded by Darwin, and commonly 

 known as the Darwinian theory — is based upon the following 

 fundamental propositions : — 



1. The progeny of all species of animals and plants exhibit 

 variations amoftgst themselves in all parts of tlieir organisa- 

 tion ; no two individuals being exactly alike. In other words, 

 in every species the individuals tend by variation to diverge 

 from the parent-type, in some particular or other. 



2. These variations can be transmitted to future generations 

 under certain definite and discoverable laws of inheritance. 



3. By artificial selection and breeding from individuals pos- 

 sessing any particular variation, man, in successive generations, 

 can 'produce a breed in which the variation is permanent; the 

 races thus produced being often as widely different as are 

 distinct species of wild animals. • 



4. The world in which all living beings are placed is one 

 not absolutely unchanging, but is liable to subject them to very 

 varying conditions. 



5. All animals and plants give rise to more numerous young 

 than can by any possibility be preserved. 



6. As these young are none of them exactly alike, a process 

 of " Natural Selection " will ensue, whereby those individuals 

 which possess any variation favourable to the peculiarities of 

 the life of the species will be preserved. Those individuals 

 which do not possess such a favourable variation will be placed 

 at a disadvantage in the "struggle for existence," and will tend 

 to be gradually exterminated. 



7. Other conditions remaining the same, the individuals 

 which survive in the struggle for existence will transmit the 

 variations, to which their preservation is due, to future gene- 

 rations. 



8. By a repetition of this process " varieties " are first estab- 

 lished ; these become permanent, and " races " are produced ; 

 finally, in the lapse of time, the differences become sufficiently 

 great to constitute distinct species. 



r4. Distribution. 



Under this head come all the facts which are concerned with 

 the external or objective relations of animals — that is to say, 

 their relations to the external conditions in which they are 

 placed. 



The geographical distribution of animals is concerned with 

 the determination of the areas within which every species of 

 animal is at the present day confined. Some species are found 

 almost everjivhere, when they are said to be " cosmopolitan]" 



