SPONTANEOUS GENERATION AND EVOLUTION : CONCLUSION 373 



sense, as the fundamental cause of the lasting' variations of oro-anic 

 forms. We can here distinguish with certainty between the direct 

 and the indirect effect of external influences, and we see how these 

 sources of variation interact upon each other. The lowest and deepest 

 root of variation is without doubt the direct effect of changed condi- 

 tions. Without this the indirect effect would have had no lever Avith 

 which to work, for the primitive beginnings of variation would 

 be absent, and an accumulation of these through personal selection 

 could not take place. It is a primitive character of living substance 

 to be variable, that is, to be able to respond to some extent to changed 

 external conditions, and to vary in accordance with them, or — as we 

 might also say — to be able to exist in many very similar but not 

 identical combinations of substances, and we must imagine that even 

 the first biophors which arose through spontaneous generation were 

 different according to the conditions under which, and the substances 

 from which, they originated. And from each of these slightly different 

 beginnings there must, in the course of multiplication by fission, have 

 been produced a whole genealogical tree of divergent variations of 

 the primitive Biophorida3, since it is inconceivable that all the 

 descendants would remain constantly under the same conditions of 

 life under which they originated. For every persistent change in 

 the conditions of existence, and especially of nutrition, must have 

 involved a variation in the constitution of the organism, whose vital 

 processes, and especially the repair of its body, depended on these 

 conditions. 



But the external influences to which the descendants of a 

 particular form of life were subject never remained permanently 

 the same. Not only did the surface of the earth and its climatic 

 conditions change in the course of time with the cooling of the earth, 

 but mountains arose and were levelled again, old land-surfaces sank 

 out of sight or emerged again, and so on ; all that, of course, played 

 its part in the transformation of the forms of life, but did so to any 

 considerable extent only at a later stage, when there were already 

 hio-hlv differentiated organisms. These unknown primitive beginnings 

 of life must have been forced to diverge into different variations 

 throuo-h the different conditions of the same place in which they 



lived. 



Let us think of the simplest microscopic Monera on the nuid of 

 the sea-coast, equipped with the faculty of plant-like assimiUxtion, 

 and we shall see that their unlimited multiplication would cause 

 differences in nutrition, for those lying uppermost would be in 

 a stronger light than those below, and would, therefore, be better 



