196 THE EVOLUTION THEORY 



if these variations come about with exceeding slowness, and are thus 

 in a certain sense ' difficult.' 



Now that I understand these processes more clearly, my opinion 

 is that the roots of all heritable variation lie in the germ-plasm, and 

 furthermore, that the determinants are continually oscillating hither 

 and thither in response to very minute nutritive changes, and are 

 readily compelled to variation in a definite direction, which maj- 

 ultimately lead to considerable variations in the structure of the 

 species, if they are favoured by personal selection, or at least if they 

 are not suppressed by it as prejudicial. But selection is continually 

 keeping watch over both kinds of variation, and if the conditions 

 of life do not further the variation or do not even allow it to persist, 

 selection eliminates everything that lessens the purity of the specific 

 type, everything that transgresses the limits of utility, or that might 

 endanger the existence of the species. Thus we understand how the 

 germ-plasm may be variable, and yet at the same time remain 

 unvaried for thousands of years, how it is ready and able to furnish 

 7 any variation that is possible in a species if that is required by 

 external circumstances, and yet is able to preserve the characters 

 of the species in almost absolute constancy through whole geological 

 ages ; in short, how it can be at once readily varial)le and yet slow 

 to \&xy. 



The importance which amphimixis thus has in connexion with the 

 adaptation of organisms lies, if I mistake not, in the necessity for 

 co-adaptation, that is, in the fact that in almost all adaptations it is 

 not merely a question of the variation of a single determinant, but of 

 the correlated variations of manj^ — often ^■ery numerous — determinants,, 

 of ' harmonious adaptation,' as we have already said. Many-sided 

 adaptation of this kind seems to me impossible without a continually 

 recurrent sifting and recombining of the germ-plasms, and this can 

 only be effected by amphimixis. 



It may be objected that, apart from amphimixis, variation can 

 be brought about in many parts of an organism, as in purely asexual 

 reproduction. A plant, for instance, may XBxy when it is transferred 

 to a strange soil or climate ; and even in that case the variations 

 seem to be harmonious, at least the harmony of the parts is so far 

 maintained that the plant continues to flourish, at any rate under 

 cultivation. A plant species may be incited by abundant nourish- 

 ment to gigantic growth, and caused to vary in many of its parts, and 

 y the almndant food may even directly affect the germ-plasm so that 

 all or some of these variations may become hereditary ; and yet this 

 is far from being a case of adaptation, it is merely a case of 



