78 Transactions. — Zoology. 



to say, by a natural and gradual modification of character, 

 due to the survival of the fittest in the universal struggle for 

 -existence. 



The principle of natural selection is expressed by Darwin 

 himself as that of " the preservation during the battle of life of 

 varieties which possess any advantages in structure, constitu- 

 tion, or instinct." He says, and with great force, " In 

 scientific investigations it is permitted to invent any hypo- 

 thesis, and if it explains various large and independent masses 

 of facts it rises to the rank of a well-grounded theory. . . . 

 If the principle of natural selection does explain these and 

 other large bodies of facts, it ought to be received. On the 

 ordinary view of each species having been independently 

 created we gain no scientific explanation of any one of these 

 facts. We can only say that it has so pleased the Creator to 

 command that the past and present inhabitants of the world 

 should appear in a certain order and in certain areas ; that 

 He has impressed on them the most extraordinary resem- 

 blances, and has classed them in groups subordinate to groups. 

 But by such statements we gain no new knowledge ; we do 

 not connect together facts and laws ; we explain nothing." * 

 In his " Origin of Species " Mr. Darwin has shown that all 

 organic beings, without exception, tend to increase at a very 

 high ratio, and that the inevitable result is an ever - recurrent 

 struggle for existence, in the natural course of which the 

 strongest ultimately prevail and the weakest fail. By this 

 process those variations, however slight, which are favourable 

 are preserved or selected, and those which are unfavourable 

 are destroyed. This continued production of new forms 

 through natural selection inevitably leads to the extermina- 

 tion of the older and less improved forms, these latter being 

 necessarily intermediate in structure, as well as in descent, 

 between the last -produced forms and their original parent 

 species. The position to which this brings us is thus stated : 

 " Now, if we suppose a species to produce two or more varie- 

 ties, and these in the course of time to produce other varieties, 

 the principle of good being derived from diversification of 

 structure will generally lead to the preservation of the most 

 divergent varieties ; thus the lesser differences characteristic 

 of varieties come to be augmented into the greater differences 

 characteristic of species, and, by the extermination of the 

 older intermediate forms, new species end by being distinctly 

 defined objects. Thus, also, we shall see how it is that 

 organic beings can be classed by what is called a natural 

 method in distinct groups — species under genera, and genera 



* " The Variations of Animals and Plants under Domestication," 

 2nd ed., vol. i., page 9, 



