30O NATURAL HISTORY. 



to do with the production of species namely, the law of 



variation and the law of inheritance. Our ignorance as 



to both of these is freely and fully admitted by Mr 



Darwin. The theory of natural selection does not profess 



to explain why variations occur ; it only explains how 



those variations which are useful to the individual are 



preserved, and how those which are injurious are ' rigidly 



destroyed.' Like all other hypotheses as to the origin 



of species, it leaves us entirely in the dark as to the 



causes of variability. The law of variation is therefore an 



unknown law, lying behind the law of evolution, and 



possibly beyond the limits of scientific investigation. 



Similarly, the laws of inheritance are almost wholly 



unknown. 'No one can say why the same peculiarity 



in different individuals of the same species, or in different 



species, is sometimes inherited and sometimes not so ; 



why the child often reverts in certain characters to its 



grandfather or grandmother or more remote ancestors; 



why a peculiarity is often transmitted from one sex to 



both sexes, or to one sex alone, more commonly but not 



exclusively to the like sex' (' Origin of Species,' page 10). 



That ' species ' have originated by modifications through 



descent may now be taken as an accepted doctrine in 



modern zoology. It is Mr Darwin's supreme merit to 



have brought about this radical change in the views of 



naturalists by the establishment of the law of 'natural 



selection,' which for the first time rendered possible an 



explanation of the method in which the modifications of 



specific forms are caused. Whether or not natural 



selection has been * the exclusive means of modification ' 



is a point upon which different naturalists hold different 



