374 THE EVOLUTION THEORY 



nourished, and, consequently, would transmit the variations thus 

 induced to their progeny which arose by fission. Thus it is conceivable 

 that even the more or less favourable position as regards light would 

 bring about the origin of two different races from the same parent 

 form, and as it is conceivable in the case of light, so is it also in 

 regard to all the influences which cause variation in the organism. 



We have already seen that variations in the lowest (non-nucleated) 

 forms of life caused by the direct influence of the vital processes may 

 be directly transmitted to the descendants, but that in all those whose 

 bodies have already differentiated into a germ- or idioplasmic-substance, 

 in contrast to a somatic substance in the more restricted sense, 

 this hereditary transmission is only possible in the case of the 

 variations of the germ-plasm, and hereditary variations of the species 

 can only arise by the circuitous route of influencing the germ-plasm. 

 The body (soma) can be caused to change by external influences, 

 by the use or disuse of an organ, but variations of this kind are not 

 transmitted ; they do not become a lasting possession of the species, 

 but cease with the individual ; they are transient changes. 



Thus it was only througli those external influences — including 

 those from the soma of the organism itself — which affected the germ- 

 substance, either as a whole or in certain of its primary constituents, 

 that hereditarily transmissible variations of the organism arose, and 

 we have already discussed in detail how j^articular variational 

 tendencies may arise through the struggle of the parts within the 

 germ-plasm, which may give an advantage to certain groups of 

 primary constituents. And these tendencies are of themselves 

 sufficient to cause the specific type to vary further and further in 

 given directions. 



Nevertheless, the infinite diversity of the forms of life could 

 never have been brought about in this way alone, if there had not 

 been another — the indirect — effect of the changeful external influences. 



This is due to the fact that the variations of direct origin sooner 

 or later obtain an influence in determining the viability of their 

 possessors, either increasing or diminishing it. It is this, in association 

 with the unlimited multiplication of individuals, which gives a basis 

 to the principle of transformation, which it is the immortal merit 

 of Charles Darwin and Alfred Russel Wallace to have introduced 

 into science: the p?'Mic^/)?e of selection. We have seen that this 

 principle may have a much more comprehensive meaning tlian was 

 attributed to it by either of these two naturalists ; that tliere is 

 not merely a struggle between individuals which brings about their 

 adaptation to their environment, by preserving those which vary in 



