302 STUDIES, SCIElsmTIC AND SOCIAL 



Suj^posed Necessitij of slwiving the Cause of Variation. 



Another objection which is dwelt upon with constant 

 reiteration by Mr. Darwin's critics is, that he has not 

 shown the cause of variation, and that whatever it is that 

 causes variation, that is the real " origin of species." This 

 has always seemsd to me one of the most unmeaning and 

 irrational of objections, because every ex]jlanation must 

 take as a basis well-known facts to explain obscure phe- 

 nomena. When the geologist explains how the contour 

 of a country has been formed by rain and ice, it is not 

 said that he has explained nothing unless he goes on to 

 show exactly how rain and snow are formed, or even goes 

 further back to the cause of gravitation which is really 

 what gives them all their power to do any work ; and 

 when the physicist explains how thunder and lightning 

 are produced by a reference to the electric spark and its 

 accompanying sound, he is not told that the explanation 

 is valueless till he has discovered the nature and cause of 

 electricity itself 



But we may, I think, go further, and say that variation 

 is an ultimate fact of nature, and needs no other explanation 

 than a reference to general principles which indicate that 

 it cannot fail to exist. Does any one ask for a reason why 

 no two gravel-stones or beach-pebbles, or even grains of 

 sand, are absolutely identical in size, shape, surface, colour, 

 and composition ? When we trace back the complex 

 series of causes and forces that have led to the production 

 of these objects, do we not see that their absolute identity 

 would be more remarkable than their diversity ? So, when 

 we consider how infinitely more complex have been 

 the forces that have produced each individual animal or 

 plant, and when we know that no two animals can possibly 

 have been subject to identical conditions throughout the 

 entire course of their development, we see that perfect 

 identity in the result would be opposed to everything 

 we know of natural agencies. But variation is merely 

 the absence of identity, and therefore requires no 

 further explanation ; neither do the diverse amounts of 



