at work.  It tells us that in searching for these forces we must look for those which have been acting constantly.  We must look for forces which produce their effects not by sudden additions to the complication of the machine.  They must be constant forces whose effect at any one time is comparatively slight, but whose total effect is to increase the complexity of the machine.  They must be forces which produce new types through the modification of the old ones.  We must look for forces which do not adapt the machine for its future, but only for its present need.  Each step in the history has been a complete animal with its own fully developed powers.  We are not to expect to find forces which planned the perfect machine from the start, nor forces which were engaged in constructing parts for future use.  Each step in the building of the machine was taken for the good of the machine at the particular moment, and the forces which we are to look for must therefore be only such as can adapt the organisms for its present needs.  In other words, nothing has been produced in this machine for the purpose of being developed later into something of value, but all parts that have been produced are of value at the time of their appearance.  We must, in short, look for forces constantly in action and always tending in the same direction of greater complexity of structure.

Is it possible to discover these forces and comprehend their action?  Before the modern development of evolution this question would unhesitatingly have been answered in the negative.  To-day, under the influence of the descent theory, stimulated, in the first place, by Darwin, the question will be answered by many with equal promptness in the affirmative.  At all events, we have learned in the last forty years to recognize some of the factors which have been at work in the construction of this machine.  We must turn, therefore, to the consideration of these factors.

==Forces at Work in the Building of the Living Machine.==—­There are three primary factors which lie at the bottom of the whole process.  They are—­

1. Reproduction, which preserves type from generation to generation.

2. Variation, which modifies type from generation to generation.

3. Heredity, which transmits characters from generation to generation.

Each must be considered by itself.

==Reproduction.==—­Reproduction is the primary factor in this process of machine building, heredity and variation being simply phases of reproduction.  The living machine has developed by natural processes, all other machines by artificial methods.  Reproduction is the one essential point of difference between the living machine and the others which has made their construction by natural processes a possibility.  What, then, is reproduction?  Reproduction is in all cases at the bottom simple division.  Whether we consider the plant that multiplies by buds or the