and beneficial qualities happen to arise as slight variations from the ancestral type, they will (other things permitting), be seized upon by natural selection, and being transmitted by heredity to subsequent generations, will be added to the previously existing type. This then, is natural selection or the survival of the fittest, "the one term referring mainly to the process, the other to the result."

The process is analogous to that by which the gardener and the cattle-breeder bring about their wonderful results. Just as these men, by always "selecting" their best individuals to breed from, slowly but continuously improve their stock, so Nature by a process of "selection," slowly but continuously makes the various species of plants and animals better suited to the conditions of their life. What the skill of Luther Burbank has accomplished in the course of a few generations, Nature takes years or even centuries of experimentation to produce. By artificial selection, man works on external characters irregularly and imperfectly for a short time. Nature works on the whole machinery of life by consistent accumulation during whole geological epochs. Silently and insensibly working, natural selection is daily and hourly scrutinizing the slightest variations, "rejecting those that are bad, preserving and adding up all that are good."

Under natural conditions there is an endless range of variation. We have seen in chapter I, how like tends to beget like, but that although the offspring is similar to the parent there is never precise reduplication. There is latitude allowed for individual variation. The individual differences are due to age, sex, modification, and

6 Thomson & Geddes, op. cit., p. 156.

real germinal variation. Whatever its cause, as long as the variation gives advantage in the struggle, the individual which possesses it, has a greater chance to survive, and surviving, to transmit it to his offspring. Occasionally, characters seem to go together in bundles; as such, they are often of advantage and are inherited. Some variations from the general type of the race are not transmitted. Variations of the mutation kind are inherited. If then, a mutation gives advantage to the individual possessing it, that individual will most probably survive while others not possessing the favorable trait will be at a disadvantage. But survival means not simply the fact of a safe and unhindered enjoyment of life. It means the bearing and rearing of young. Biologically, survival means that the individual reaches maturity and has offspring to which he transmits the favorable characteristics that aided him in the struggle.

The struggle will usually be "most severe between individuals of the same species, for they frequent the same districts, require the same food, and are exposed to the same dangers," In such a case the most minute variation may determine which will survive. As many variations seem to be the result of pure chance, so survival is in many instances the result of pure chance. An illustration will make this clear. Dr. C. B. Davenport of the Carnegie Institution for Experimental Evolution, placed 300 chickens in an open field. Eighty per cent. were white or black and hence conspicuous; 20 per cent. were spotted and hence inconspicuous. In a short time twenty-four were killed by crows, but only one of the killed was spotted. The white and black chickens were easily discernible to the crows and hawks flying overhead, and they swooped down and carried off twenty

three. The spotted chickens were not so easily seen from above. Only one of them was killed. Thus the mere chance of coat color was a decisive factor in determining which chickens should survive. In time it is probable that more of the black and white chickens would be killed by birds of prey and only the spotted chickens would be left. Their offspring would tend to inherit their spotted coat and hence survive. All offspring which varied from this type in the direction of having a white coat or a black coat would be likely to be killed and leave no black or white coated progeny. Eventually we might have only a spotted variety of chickens in this area. This is an excellent illustration of the principle of natural selection.

Where the characters of an inhabiting species show great variation we conclude that there has not been rigorous selection with reference to that character. That is, the trait in question is one which, at the time being, is neither of great disadvantage nor great advantage to those who possess it. It was originally acquired or preserved because it was favorable, but some change has been wrought which makes it of indifferent value. On the other hand if any character shows very slight variation as between a large number of the species, we conclude that selection with reference to it has been severe; that is, the trait in question gives positive advantage. Thus, before the coming of the crows, coat color in chickens was of indifferent value for survival and there was wide variation from white to black. But with the coming of the birds of prey, conditions were changed and coat color had a positive survival value, if it were inconspicuous. The wide variation soon disappeared (the black and white chickens were killed off)

and all the chickens had a spotted coat. So by recording the variation in any trait we can tell whether it has a survival value and whether natural selection has caused its uniformity. This is the reason why we find that all the individuals of any species are more or less alike, because variation outside of certain safe limits is hazard

ous.

A rain storm once washed a large number of sparrows out of their nests. An observer gathered the injured sparrows together and tried to revive them. A large number of the birds recovered but some did not survive. Measurements of all the dead and revived sparrows were taken, and the curve showing their distribution (the frequency with which each measure occurred) was plotted. Then the measurements of the revived sparrows were taken separately, and their distribution was plotted. It was found that the measurements of the surviving sparrows varied less from the average degree of the character measured than did the measurements of the dead sparrows; that is, the dead sparrows were more variable. The curve representing the birds which survived was a narrower and steeper curve, which showed that the birds killed were more largely the unusual, the extreme, those widely differing from the average. We find pretty generally that the extreme variates from the normal are less likely to survive the dangers of their surroundings. The more normal are more likely to survive.

By examining the smooth curve in figure 2, which represents the approximate distribution of the frequencies at which different heights occur among ten and one-half year old American boys, one can better understand this normal order. If there should be an epidemic of scarlet fever and all of the boys contracted the disease, 30

might die. Probably the boys whose stature is between 145 and 153 cm., and hence considerably above the normal (which is between 129 and 137 cm.), are boys who have outgrown their strength. On the other hand, the boys whose stature is between 109 and 117 cm., and hence considerably below the normal, are boys who have been stunted perhaps because of constitutional weakness. This group of extremely tall and extremely short boys (for the age of 1011⁄2 years) is more likely, other things being equal, to succumb to the disease, than the more normal individuals; that is, outgrown strength and anemic condition in the one case, and weak constitution or lack of nourishment on the other, constitute conditions which break down the power of resistance to disease. It is therefore possible that most of the 30 cases of mortality would be found among this group of very short and very tall boys. If now, the curve were plotted for the remaining 970 boys who survived, it would be found that the curve was narrower than before, that is, that the falling away to the right and left had disappeared. In some such way as this, Nature tends to cut off the extreme variates and to reduce the race or species to a certain uniformity.

The plant and animal organism is a plastic, changing thing. It readily adapts itself to new situations. A sudden change in climate becoming a permanent condition of a given locality will affect all forms of life in that locality. Some individuals will not have sufficient adaptability to adjust themselves to the new requirements of their surroundings; they will sicken and die. Those individuals who chance to be plastic enough to meet the change by new adjustments in their habits and mode of life, will most probably survive and pass some