The horse is in many ways a remarkable animal; not least so in the fact that it presents us with an example of one of the most perfect pieces of machinery in the living world. In truth, among the works of human ingenuity it cannot be said that there is any locomotive so perfectly adapted to its purposes, doing so much work with so small a quantity of fuel, as this machine of Nature's manufacture--the horse. These are but a few out of countless similar examples. "We are constantly discovering," says Lord Grimthorpe, "new complications and processes, and what to all common sense appear contrivances, in the organs of all living things, and indeed we can find no limit to them." In all these cases an instrument is fashioned precisely adapted to the performance of a certain function, and it is therefore obvious on first principles that there must exist _some_ power capable of producing such instruments. It will probably be answered that there are forces enough in Nature to account for everything, and that these furnish the needful explanation. But, as Mr. Croll rightly insists,[135] Force by itself explains nothing. Its mere exercise has no tendency whatever to produce such effects. There must likewise be Determination of Force in the one definite direction required, and it is in the source of this Determination that the true cause must be sought to which the result is due. It is not simply because iron is hammered and filed that a railway-engine is produced; nor is it sufficient that a block of marble be chipped with mallet and chisel in order to obtain a statue of Apollo. Unless some influence comes in to direct the forces in such cases to their respective results, the results will never by any possibility be secured. And in the processes of Nature such direction or determination must be exercised in particulars inconceivably intricate, to which the works of man furnish no parallel. As Mr. Croll writes: If a tree is to be formed, the lines of least resistance must all be determined and adjusted in relation to the objective idea of the tree; of the root; of the branches; of the leaves; of the bud; of the fruit; and of every part of the tree. But this is not all: the tree is built up molecule by molecule, each of which requires a special determination, and, beyond all this, we have the structureless protoplasm, which must be differentiated according to the objective idea of the whole. What produces this marvellous adjustment of means to ends? And as he insists in another passage: