or as a result of sexual combination of germ plasm, shows a variation from its parents.  This variation proves of value to its possessor, who lives and transmits it permanently to posterity.  Thus step by step, one part is added to another, until the machine has grown into the intricately adapted structure which we call the animal or plant.  This has been nature’s method of building machines, all based upon the three properties possessed by the living cell—­reproduction, variation, and heredity.

==Summary of Nature’s Power of Building Machines.==—­Let us now notice the position we have reached.  Our problem in the present chapter has been to find out whether nature possesses forces adequate to explain the building of machines with their parts accurately adapted to each other so as to act harmoniously for certain ends.  Astronomy has shown that she has forces for the building of worlds; geology, that she has forces for making mountain and valley; and chemistry, that she has forces for building chemical compounds.  But the organism is neither a world, nor a mass of matter, nor a chemical compound.  It is a machine.  Has nature any forces for machine building?  We have found that by the use of the three factors, reproduction, variation, and heredity, nature is able to produce a machine of ever greater and greater complexity, with the parts all adapted to each other.  Now the difference between a machine and a mass of matter is simply in the adaptation of parts to act harmoniously for definite ends.  Hence if we are allowed these three factors, we can say that nature does possess forces adequate to the manufacture of machines.  These forces are not chemical forces, and the construction of the machine has thus been brought about by forces entirely different from those which produced the chemical molecule.

But we have plainly not reached the bottom of the matter in our attempt to explain the machinery of living things.  We have based the whole process upon three factors.  Reproduction, variation, and heredity are the properties of all living matter; but they are not, like gravity and chemism, universal forces of nature.  They occur in living organisms only.  Why should they occur in living organisms, and here alone?  These three properties are perhaps the most marvellous properties of nature; and surely we have not finished our task if we have based the whole process of machine building upon these mysterious phenomena, leaving them unintelligible.  We must therefore now ask whether we can proceed any farther and find any explanation of these fundamental powers of the living machine.