THE CAUSE OF VARIATION 87 



ties of thought. As long ago as 1689 Locke referred our ideas 

 of the primary qualities of bodies to the " bulk, figure, number, 

 situation, and motion of their parts." Since his day we have 

 learnt much about these " parts," and in the light of modern 

 knowledge we may paraphrase his words by the expression 

 " kind, number, and arrangement of the component molecules," 

 because the "bulk and figure" of these depend on their kind, 

 which also determines their motion and the manner of their 

 response to the motions reaching them from the environment. 

 And as the primary qualities of a body are referable to these 

 factors, so also must the differences between any two be referred 

 to differences between these factors. It would appear moreover 

 that the analysis may be carried one step further, resolving the 

 factor of arrangement into the two others. For in the growth 

 of organic matter the positions taken up by the molecules must 

 depend upon their natures and numbers. The differences 

 between any two like individuals are therefore results of the 

 differences in the numbers of the chemical molecules in the 

 cells they grew from — a proposition which, I may perhaps 

 be told, might have been assumed at the outset. An important 

 consequence should however be noted. As the differences 

 between any two such cells consist only in differences in the 

 numbers of the molecules composing them, it follows that 

 all variations must be in one or other of two directions— plus 

 or minus, greater or less — and it is inaccurate and misleading, 

 though very common, to write of them as occurring " all round 

 a centre," " in every possible direction." They must in every 

 particular be above or below a mean, and there is no third 

 alternative. 



When the matter is thus considered in a general way, it 

 becomes evident that all that has been said applies not merely 

 to living bodies, but to all bodies, organic and inorganic alike. 

 Locke's words are of course perfectly general. Mountains, 

 rivers and clouds, as well as animals and plants, show striking 

 general resemblances accompanied by endless differences in 

 detail. Not only are no two leaves alike on a tree, but the same 

 may be said of the pebbles on the beach, and with the aid of 

 a microscope the truth is found to hold even of the grains 

 of sand on which they lie. Many rocks look homogeneous 

 enough, but on being examined in thin plates under high 

 powers they reveal a complicated intimate structure. Organic 



