This is what happens. First the leaves are eaten from all the lower branches as they are reached with the least effort. Then they go higher and still higher until the point is reached where number one with the shortest neck cannot reach any further and the terrible struggle for existence begins. Number two sees no danger as yet and number three has things all his own way. But with short-necked number one, a tragedy has begun. Every day now sees the food further out of his reach and even number two is obliged to reach out for his supply. The breeding time is approaching but the longer necked and therefore well-fed and vigourous females will have nothing to do with this wobbley starving creature, and the longer necked, well-fed males shun the short-necked starving females. If the starving ones mate, the mother dies before giving birth to offspring, or she cannot get nourishment enough to rear her progeny; in either case there is no effective succession. So the longer-necked are the fittest and they survive. Thus does nature “select” one by the negative process of destroying the rest, in about the same way as a man “selects” one puppy in a litter by drowning the rest.
In the case of the puppies we may say “artificial selection;” in the case of the giraffe it is “natural selection.” And this theory, simple as it may seem here, revolutionized Biology.
It is worthy of note that “natural” selection has many advantages over “artificial” selection. The breeder may be mistaken; he may select the wrong puppy and drown its superior. The horse that won the great race may have had a fleeter-footed companion in the same stable had the trainer known how to develop his possibilities. The gardener may have passed the best root or stem through carelessness. But nature makes no such mistakes, or if she does she eventually redeems them. Her method, while it is wholly fortuitous and unintelligent, is practically infallible. The condition of survival is, adaptation to environment. The very process of selection is, in itself, a sure test of fitness. True, moral considerations are eliminated--at least in the non-social world--yet nature offers something like a fair field and no favors. When we speak of nature’s favorites, we simply mean those who are best fitted to meet her hard conditions.