Thus, although none of the whiteness due to somatic variations is transmitted to the offspring, such variations are of considerable importance to the species, as they enable it to survive and allow time for the germinal variations in the required direction to appear. That this case need not be purely hypothetical is shown by the fact that dun domestic pigeons, which are of an earthy-brown colour when fresh moulted, soon fade in the sun to a dull creamy hue. Thus a coloration adapted to an ordinary soil could soon be suited to a desert environment. The ruddy sheldrake also, normally a bright chestnut-coloured bird, and one that haunts exposed sunny places, in many cases fades very much, becoming almost straw-coloured. Many variations which organisms display are of a mixed kind, being in part the result of inner forces and in part due to the action of the environment. In so far as they are due to this latter they do not appear to be inherited. Thus, although we cannot say of many variations whether they are germinal, or somatic, or of a mixed kind, it is of great importance to keep continually in mind the fundamental differences between the two kinds. Some somatic variations are due to the direct action of the environment; they are merely the expression of the manner in which an organism responds to external stimuli. What is the cause of germinal variations? This is a question to which we are not yet in a position to give a satisfactory answer. The attempt to explain their origin plunges us into the realm of theory. This doubtless is a realm full of fascination, but it is an unexplored region of extreme darkness, in which, we believe, it is scarcely possible to take the right road until more of the light of fact has been shed upon it. In the chapter dealing with inheritance we shall indicate the lines along which it is likely that future progress will be made. CHAPTER IV HYBRIDISM The alleged sterility of hybrids a stumbling-block to evolutionists—Huxley’s views—Wallace on the sterility of hybrids—Darwin on the same—Wallace’s theory that the infertility of hybrids has been caused by Natural Selection so as to prevent the evils of intercrossing—Crosses between distinct species not necessarily infertile—Fertile crosses between species of plants—Sterile plant hybrids—Fertile mammalian hybrids—Fertile bird hybrids—Fertile hybrids among amphibia—Limits of hybridisation—Multiple hybrids—Characters of hybrids—Hybridism does not appear to have exercised much effect on the origin of new species.