But how, it is asked, can we accept it if its _modus operandi_ is inexplicable? I question the validity of this argument. I fear our knowledge of organic nature is not at present so full and exact as to justify us in excluding an hypothesis because we are not able to give an adequate answer to the question--How are these effects produced? Of course, if it can be shown that no _modus operandi_ is possible, there is an end of the matter. But who shall dare thus to limit the possibilities of organic nature? And, if possible, then that natural selection in which the neo-Darwinians place their sole trust would certainly develop so advantageous a mode of influence. It is clear that a species sensitive to every shock of the environment on the organism would be unstable, and hence at a disadvantage. But, on the other hand, the ability to answer by adaptation to long-continued and persistent environmental influence or to oft-repeated and consistent performance of function would be so distinct an advantage to the species which possessed it, that, if it lay within the possibilities of organic nature, natural selection, always, as we are told, on the look out for every possible advantage, would assuredly seize upon it and develop it. Those who believe in the absolute sway of natural selection have not at present given any adequate answer to the question--How are particular variations (e.g. the twisted skull of flat-fish) produced? They say that constitutional variations, which are alone inheritable, are due to variations in the germs. When asked how these variations are produced, they are forced to reply--We cannot say. But when it is suggested that they may be in some unknown way transmitted to the germ from the body, they are up in arms, and exclaim--You have no right to believe that, or ask us to believe it, unless you can tell us plainly how the effect is produced. Unable themselves to give the _modus operandi_ of the origin of particular variations, they demand the exact _modus operandi_ from those who suggest that variations may arise through this mode of influence of the body on the germ. We shall have to consider this question from a more general standpoint in the next chapter on "Organic Evolution." We may now very briefly summarize some of the results we have reached in this chapter. The ova and sperms are specially differentiated cells which have, in the division of labour, retained and emphasized the function of developmental reproduction. There is a continuity of such cells. The cells which become ova or sperms have never become differentiated into anything else. Hereditary similarity is due to the fact that parents and offspring are derived eventually from the same germinal cells. Variation in the existing world is partly due to sexual union. But if there be mere admixture, new characters cannot arise in this way, nor can old characters be strengthened beyond the existing maximum.