48 MANUAL OF ZOOLOGY. 



though all are not agreed as to the manner in which that 

 evolution has been carried out. 



I. Doctrine of Special Creation. On this doctrine of the 

 origin of species it is believed that species are immutable pro- 

 ductions, each of which has been specially created at some 

 point within the area in which we now find it, to meet the 

 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 organisation, 

 no two individuals being exactly and in all respects alike. In 

 other words, in every species the individuals, whilst inheriting 

 a general likeness to their progenitors, tend by variation to 

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



2. Variations arising in any part of the organism, however 

 minute, may be transmitted to future generations, under certain 

 definite and discoverable laws of inheritance. 



3. By " artificial selection," or by breeding from individuals 

 possessing any particular variation, man, in successive genera- 

 tions, can produce a breed in which the variation will be per- 

 manent, the divergence from the parent-type being usually 

 intensified by the process of inter-breeding. The races thus 

 artificially produced by men are 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, on the contrary, 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, each species tending 

 to increase in numbers in a geometrical progression. 



6. As these young are none of them exactly alike in all 



