328 THK INDUCTIONS OF BIOLOGY. 



differences thus initiated become increasingly pronounced. 

 Among young plants the smaller, outgrown by their better- 

 placed neighbours, are continually more shaded and more left 

 behind; and among the litter the weakly ones, continually 

 thrust aside by the stronger, become relatively more weakly 

 from deficient nutrition. 



Differentiations thus arising, both before and after separa 

 tion from parents, though primarily differences of growth, 

 entail structural differences ; for it is a general law of nutri 

 tion that when there is deficiency of food the non-essential 

 organs suffer more than the essential ones, and the unlikc- 

 nesses of proportion hence arising constitute unlikenesses of 

 structure. It may be concluded, however, that variations 

 generated in this manner usually have no permanent results. 

 In the first place, the individuals which, primarily in growth 

 and secondarily in smaller developments of less-important 

 organs, are by implication inferior, are likely to be eliminated 

 from the species. In the second place, differences of struc 

 ture produced in the way shown do not express differences of 

 constitution are not the effects of somewhat divergent 

 physiological units; and consequently are not likely to be 

 repeated in posterity. 



88. We have still, therefore, to explain those variations 

 which have no manifest causes of the kinds thus far con 

 sidered. These are the variations termed &quot; spontaneous.&quot; 

 Not that those who apply to them this word, or some equi 

 valent, mean to imply that they are uncaused. Mr. Darwin 

 expressly guards himself against such an interpretation. He 

 says : &quot;I have hitherto sometimes spoken as if the varia 

 tions so common and multiform in organic beings under 

 domestication, and in a lesser degree in those in a state of 

 nature had been due to chance. This, of course, is a wholly 

 incorrect expression, but it serves to acknowledge plainly our 

 ignorance of the cause of each particular variation.&quot; Not 

 only, however, do I hold, in common with Mr. Darwin, that 



