NEWTON AND DARWIN. 25 
value, the more safely we are guided by it, and the more 
strongly are we bound to adopt it. 
Let us call to mind, for example, that theory which has 
ranked up to the present time as the greatest achievement 
of the human mind—the Theory of Gravitation, which 
Newton, two hundred years ago, established in his Mathe- 
matical Principles of Natural Philosophy. Here we find 
that the object to be explained is as large as one can well 
imagine. He undertook to reduce the phenomena of the 
motion of the’planets, and the structure of the universe, to 
mathematical laws. As the most simple cause of these in- 
tricate phenomena of motion, Newton established the law 
of weight or attraction, the same law which is the cause of 
the fall of bodies, of adhesion, cohesion, and many other 
phenomena. 
If we apply the same standard of valuation to Darwin’s 
theory, we must arrive at the conclusion that this theory, 
also, is one of the greatest achievements of the human mind, 
and that it may be placed quite on a level with Newton's 
Theory of Gravitation. Perhaps this opinion will seem a 
little exaggerated, or at any rate very bold, but I hope in 
the course of this treatise to convince the reader that this 
estimate is not too high. In the preceding chapter, some 
of the most important and most general phenomena in 
organic nature, which have been explained by Darwin’s 
theory, have been named. Among them are the varia- 
tions in form which accompany the individual development 
of organisms, most varied and complicated phenomena, 
which until now presented the greatest difficulties in the 
way of mechanical explanation, that is, in the tracing of 
them to active causes. We have mentioned the rudimen- 
