24 



THE DIFFEEENTIATION OF SPECIES. 



Summary of 

 the facts of 

 Evolution. 



The laws of 

 Evolution 

 only apply 

 to organic 

 matter. 



which seems to be the only possible key to the explanation of the theory of evolution by 



DESCENT WITH MODIFICATION, and DIFFERENTIATION BY MODIFICATION IN ISOLATION. 



We may sum up the known facts of Evolution as follows : — All birds are descended 

 from semireptilian ancestors, most probably from one species of semireptile. Each genera- 

 tion slightly varied from the preceding one until the species became exactly adapted to its 

 environment. The variation was most rapid where the need of variation was greatest, and 

 trivial or imperceptible where no need for it existed. The variations were not accidental, 

 because they occurred simultaneously, in a number of individuals, in the same locality and 

 continued for many generations. The rapidity with which the modifications accumulated 

 was constantly increased by the greater longevity and consequent prolificness of the 

 individuals which had acquired them, and constantly retarded by their interbreeding with 

 those which had not, the two causes combined preventing differentiation. The causes of 

 variation appear to be both subjective and objective ; birds appear to have modified their 

 organs by use or disuse, or to have had them modified in some unknown way by surrounding 

 circumstances. The causes of differentiation are Isolation either of time or space : of time, 

 causing the birds of an island to differ specifically from their ancestors of a hundred 

 thousand years ago ; of space, causing the birds of one island to differ specifically from 

 those of another. The principal cause of geographical isolation has been emigration, and 

 the principal necessity to emigrate has been overcrowding of population, frequently produced 

 by the contraction of the area of distribution by a glacial epoch. 



It seems probable that not only all animals, but also all plants, had a common origin, 

 and that the original monad developed, and its descendants became differentiated, in the 

 course of ages according to the laws of Evolution into the multitudinous species which now 

 represent organic life on our world. There is not, however, a shred of evidence to be found 

 in favour of the theory that the laws of Evolution apply in the least degree to inorganic 

 matter. On the contrary, there is every reason to believe that the species of inorganic 

 matter, or, as they are usually called, the elements of matter, have always been differentiated 

 and will always continue so without a shadow of modification, crossbreeding and inter- 

 breeding in every possible combination notwithstanding. We may temporarily modify 

 them by heat or by some other force, but they inevitably revert back to their original 

 condition, if placed in similar circumstances. Every known fact bearing upon the question 

 supports the theory that the laws of Evolution apply to all organic life, and have nothing 

 whatever to do with inorganic matter. The gradual development of the solar system, from 

 a widely diffused nebulous matter to its present condition, may have presented a series of 

 changes somewhat similar to those which have taken place in the development of organic 

 life ; but if we apply the term Evolution to the latter process, it would be a great mistake 

 to apply it also to the former, which has proceeded on divergent lines, governed by different- 

 laws, and probably tending to quite other ends. 



