There may very probably be a struggle for existence between organs or cells in the body of each individual.  The amount of nutriment in the body is a more or less fixed quantity; and if one organ seizes more than its fair share, others may or must diminish for lack.  But the limit to this usurpation must apparently be set by the crowding out of those individuals in which it is carried too far.  Natural selection, so to speak, leaves the individual responsible for the distribution of the nutriment among the organs, and spares or destroys the individual as this usurpation proves for its advantage or disadvantage.

It makes its verdict much as the judges at a great poultry or dog show count the series of points, giving each one of them a certain value on a certain scale, and then award the prize to the individual having the highest aggregate on the whole series.  Any such illustration is very liable to mislead; I wish to emphasize that fitness to survive is determined by the aggregate of the qualities of an individual.

But an animal having one organ of great value or capacity may thus carry off the prize, even though its other organs deserve a much lower mark.  This is the case with man.  In almost every respect, except in brain and hand, he is surpassed by the carnivora, the cat, for example.  But muscle may be marked, in making up the aggregate, on a scale of 500, and brain on a scale of 5,000, or perhaps of 50,000.  A very slight difference in brain capacity outweighs a great superiority in muscle in the struggle between man and the carnivora, or between man and man.

The scale on which an organ is marked will be proportional to its usefulness under the conditions given at a given time.  During the period of development of worms and lower vertebrates much muscle with a little brain was more useful than more brain with less muscle.  Hence, as a rule, the more muscular survived; the brain increasing slowly, at first apparently largely because of its correlation with muscle and sense-organs.  At a later date muscle, tooth, and claw were more useful on the ground; brain and hand in the trees.  Hence carnivora ruled the ground, and certain arboreal apes became continually more anthropoid.  At a later date brain became more useful even on the ground, and was marked on a higher scale, because it could invent traps and weapons against which muscle was of little avail.  Just at present brain is of use to, and valued by, a large portion of society in proportion to its efficiency in making and selfishly spending money.  But slowly and surely it is becoming of use as an organ of thought, for the sake of the truth which it can discover and incarnate.