2o8 HOLISM AND EVOLUTION chap. 



associations are no doubt in part due to Natural Selection, 

 but in part the physical environment has probably exercised 

 a direct pressure all its own and produced an effect which 

 has powerfully reinforced the results of Natural Selection. 

 The hereditary variation ultimately appears, but it does so 

 not accidentally or from the blue, but from the long- 

 continued stimulus of environmental conditions which have 

 influenced and affected the field of the germ-cell. 



While some variations thus have their roots in the 

 traditional use and practice of individuals or in the condi- 

 tions of the physical environment, and can survive under 

 the protection thus afforded them, many variations cannot 

 be thus accounted for, and probably originate in what ap- 

 pears to us as a spontaneous, independent, more or less 

 sudden and accidental manner. The mode of their selec- 

 tion and survival has still to be accounted for. Before 

 doing so it is advisable to mention a third set of difficulties 

 which Darwinism encounters in its explanation of organic 

 Evolution. I refer to the phenomena of co-ordination 

 and co-adaptation of organs and characters which it is al- 

 most impossible to account for satisfactorily on orthodox 

 Darwinian lines. 



I have hitherto spoken of variations as if they came 

 singly in the evolution of organisms. But they do appear 

 but rarely as single units. Generally they appear in asso- 

 ciated groups. A small variation is generally found to be 

 accompanied by a number of still smaller associated varia- 

 tions. If an organ varies, the associated muscles, nerves 

 and other body-cells undergo a corresponding variation. 

 The evolution of the horns of a wild beast, for instance, 

 means minor and consequential adjustments to its head, 

 its neck, its muscular system, the development of the fore- 

 part of the body, and its relation to the back parts, as 

 well as to many other parts and details of its body. And 



variations correspond to what has above been called modifications, while 

 mutations correspond to what has been called variations.) The experi- 

 ments of Kammerer and Durkhen on animals and plants would seem 

 to tend in the same direction. But they require further corrobora- 

 tion. 



