Now all these effects are direct modifications of the machine, and if they are only transmitted to following generations so as to become permanent modifications, they will be most important agencies in the machine building.  If, on the other hand, they are not transmitted by heredity, they can have no permanent effect.  We have here thus again the problem of the inheritance of acquired characters.  We have already noticed the uncertainty surrounding this subject, but the almost universal belief in the inheritance of such characters requires us to refer to it again.  It is uncertain whether such direct effects have any influence upon the offspring, and therefore whether they have anything to do with this machine building.  Still, there are many facts which point strongly in this direction.  For example, as we study the history of the horse family we find that an originally five-toed animal began to walk more and more on its middle toe, in such a way that this toe received more and more use, while the outer toes were used less and less.  Now that such a habit would produce an effect upon the toes in any generation is evident; but apparently this influence extended from generation to generation, for, as the history of the animals is followed, it is found that the outer toes became smaller and smaller with the lapse of ages, while the middle one became correspondingly larger, until there was finally produced the horse with its one toe only on each foot.  Now here is a line of descent or machine building in the direct line of the effects of use and disuse, and it seems very natural to suppose that the modification has been produced by the direct effect of the use of the organs.  There are many other similar instances where the line of machine building has been quite parallel to the effects of use and disuse.  If, therefore, acquired characters can be inherited to any extent, we have, in the direct influences of the environment an important agency in machine building.  This direct effect of the conditions is apparently so manifest that one school of biologists finds in it the chief cause of the variations which occur, telling us that the conditions surrounding the organism produce changes in it, and that these variations, being handed down to subsequent generations, constitute the basis of the development of the machine.  If this factor is entirely excluded, we are driven back upon the natural selection of congenital variations as the only kind of variations which can permanently effect the modification of the machine.

==Consciousness.==—­It may be well here to refer to one other factor in the problem, because it has somewhat recently been brought into prominence.  This factor is consciousness on the part of the animal.  Among plants and the lower animals this factor can have no significance, but consciousness certainly occurs among the higher animals.  Just when or how it appeared are questions which are not answered, and perhaps never will be.  But consciousness,