38 MANUAL OF ZOOLOGY. 



external conditions there prevailing, subsequently spreading 

 from this spot as far as the conditions of life were suitable 

 for it 



II. Doctrine of Development. On the other hand, it is be- 

 lieved that species are not permanent and immutable, but that 

 they " undergo modification, and that the existing forms of life 

 are the descendants by true generation of pre-existing forms." 

 (Darwin.) 



On Lamarck's theory of the development of species, the 

 means of modification were ascribed to the action of external 

 physical agencies, the inter-breeding of already existing forms, 

 and the effects of habit. 



The doctrine of the development of species by variation and 

 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 amongst themselves in all parts of their 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- 



