136 ELEMENTS OF BIOLOGY. 



expressed in the above proposition, considerable difference 

 of opinion obtains as to the viethod in which evolution has 

 been brought about. 



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

 means of modification were ascribed to the action of exter- 

 nal physical agencies, the interbreeding of already existing 

 forms, and the effects of habit, or the use and disuse of cer- 

 tain organs. 



The doctrine of the evolution of species by variation and 

 *' Natural Selection" — propounded by Mr Darwin, and com- 

 monly knoAvn as the Darwinian theory — is based upon the 

 folldii^ng fundamental propositions : — 



1. The progeny of all species of animals and plants 

 exhibit variations amongst themselves in all parts of their 

 organisation, no tvvo individuals being exactly and in all 

 respects alike. In other words, in every species the indi- 

 viduals, whilst inheriting a general likeness to their progeni- 

 tors, tend by variation to diverge from the parent type in 

 some particular or other. 



2. Variations arising in any part of the organism, how- 

 ever minute, may be transmitted to future generations, under 

 certain definite and discoverable laws of inheritance. 



3. By "artificial selection," or by breeding from indi- 

 viduals possessing any particular variation, man, in succes- 

 sive generations, can produce a breed in which the variation 

 will be permanent, the divergence from the parent type 

 being usually intensified by the process of interbreeding. 

 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 spe- 

 cies tending to increase in numbers in a geometrical 

 progression. 



